WASHINGTON -- Marco Rubio staked his presidential campaign on a prediction: Global unrest, he told the Associated Press a year ago, would cause voters to place higher-than-usual weight on the foreign policy positions of presidential candidates. Rubio would run on foreign policy, and Republican primary voters would choose him as their nominee.

Rubio kept his side of the bargain. On the campaign trail, the Florida senator said the U.S. should challenge Moscow by enforcing a no-fly zone in Syria and ramping up lethal aid to the Ukrainians, be ready to “take action” to halt China’s incursion into the disputed South China Sea, and scrap an international nuclear agreement with Iran. He said the U.S. could have won the Iraq War if only President Barack Obama, who withdrew troops in 2011, had the guts to stay there longer, and argued that the problem in Libya is that the U.S. didn’t go in soon enough, stay long enough or exert enough force. He proposed spending an additional $1 trillion on funding the military. Through the first 11 Republican debates, two of his five most-repeated talking points referred to America's place in the world, according to an analysis by FiveThirtyEight.

When they did focus on foreign policy -- which wasn't often -- Rubio's top rivals pushed a more isolationist vision. Cruz has suggested that leaving Assad in power is better than any alternative. Trump has said the U.S. should let the Russians handle the fight in Syria.

Rubio's focus on foreign policy has won him the support of many of his party's leading hawks, including former George W. Bush aides Eric Edelman, Elliott Abrams and Eliot Cohen. But it hasn’t translated into votes. He has won just three primary contests and trails Donald Trump in polls of his home state, Florida. A February YouGov poll showed that Rubio's 2015 prediction was wrong: Just 25 percent of Republican voters identify foreign policy as one of the two issues they care about most. 

Why would Rubio focus his campaign on issues only a quarter of Republican voters care about?

Some of Rubio's friends and advisers suggest he can't help focusing on foreign policy. They say his foreign policy beliefs are deeply held, a product of his childhood as the son of Cuban exiles.

Another group -- including aides who watched his work as a member of the Senate foreign relations committee -- argues that his bellicose rhetoric is a put-on, a calculated move to appeal to people who could help pave his way to the White House.

Neither answer looks good for the floundering Rubio campaign. If the first group is right, and his hawkishness is innate, Republican voters have rejected a crucial part of his identity, instead opting for the more isolationist policies of his rivals. If the second group is right, and Rubio chose to emphasize foreign policy because he thought that's what would win him the nomination, it could be the mistake that dooms his campaign.

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U.S. foreign policy was personal for Rubio from a young age. His grandfather, who fled Cuba, taught him that the U.S. had a responsibility to lift up less democratic countries. “He believed the United States was destined to be the defender of human progress, the only power capable of preventing tyranny from dominating the world,” Rubio wrote in his memoir. "I boasted I would someday lead an army of exiles to overthrow Fidel Castro and become president of a free Cuba … .”

Rubio says that the 1980 presidential election, when he was 8, was a formative event in his political education. Rubio was initially drawn to Sen. Ted Kennedy, (who lost the Democratic nomination to President Jimmy Carter), but his grandfather quickly set him straight. “Reagan was his man,” Rubio wrote.

That wasn't unusual in Rubio's community, says Nelson Diaz, a former Rubio aide. “As a member of the exile community in Miami, foreign policy is huge to all of us,” he said. “When Marco was growing up, when Cubans got together, there were basically two things they talked about: Ronald Reagan and foreign policy -- particularly as it relates to Cuba."

Rubio's grandfather told him Carter was weak, and cited the Iran hostage crisis and the Soviet invasion of Afghanistan as evidence that the rest of the world didn’t respect or fear the president. When the hostages in Iran were freed on Reagan’s inauguration day, Rubio’s grandfather, like many Republicans, credited the strength of their new president for Iran’s capitulation, ignoring the Carter administration’s diplomatic efforts in the lead-up to the prisoner release. Rubio has been a Republican ever since, he wrote.

Rubio made his entry into elected office in 1998, two years after graduating from law school, as the city commissioner for West Miami, where the most contentious issue he faced was whether to change local building code to allow construction of two-story houses. During his almost nine years in the Florida House of Representatives, including two as Speaker of the House, Rubio focused on tax policy, education and local crime.

When he considered whether to run for an unexpectedly available Senate seat in the 2010 elections, Rubio cited the “disastrous shape” of the economy as his main pull to national politics. But he was also concerned with Obama’s promises to reverse Bush-era national security policies: “closing Guantanamo, retreating from Iraq and trying terrorists in civilian courts,” he listed in his memoir. “I thought their positions were not just naive, but dangerous.”

Foreign policy was conspicuously absent from Rubio's Senate campaign. Competing against Florida’s popular moderate governor, Charlie Crist, for the Republican nomination, Rubio lurched to the right and focused on domestic issues like Obamacare and reigning in government spending. For this, he won the support of the tea party, a political movement that prizes American exceptionalism but places little emphasis on foreign policy. (Tea party Republicans now overwhelmingly support Cruz, Rubio’s rival and Senate colleague.)

But behind the scenes, Rubio was already reaching out to the hawks who would later back his presidential campaign. In 2010, while Rubio was still campaigning for the Senate, a friend put him in touch with Elliott Abrams, a George W. Bush and Ronald Reagan aide who pleaded guilty to withholding information from Congress in the Iran-Contra affair.

Though their 30-minute phone call was six years ago, Abrams remembers being “deeply impressed” with the young politician, whose resume didn’t suggest an expertise in foreign affairs. “How do you want me to talk about the Middle East?” Abrams began. “Do you want me to go geographically? I can start in Morocco and we’ll go east -- or do you want to talk about Israel first?”

“No, let’s do it this way,” Rubio said, according to Abrams. “Let me tell you what I think about events in the Middle East and then tell me where you think I’m wrong.” He spoke for seven or eight minutes, and presented an understanding of the region that Abrams described as “rather fantastic” and left him with little to add.

It was clear to Abrams that what Rubio lacked in experience, he made up for with a studious attention to world affairs. Abrams, who usually refrains from endorsing candidates because of his role at the Council on Foreign Relations, publicly threw his weight behind Rubio’s presidential campaign earlier this month.

One of Rubio’s first efforts after moving to Washington was to secure seats on the Senate foreign relations and intelligence committees, coveted positions he still holds today. But even members of his own party suspect that might have as much to do with his ambition as his interests.

“It’s not to speak ill of him, but I think he’s very ambitious, and I think he always kind of had his eyes set on doing what he’s doing now,” said Rep. Evan Jenne (R), a member of the Florida House of Representatives who was a freshman member when Rubio was speaker. “And reviewing his own portfolio, that would have been his weakness -- his lack of foreign policy experience. So to see him rush toward that as soon as he got to Washington, I think, makes complete sense.”

After initially agreeing to speak to The Huffington Post, the Rubio campaign did not respond to repeated requests for comment.

Whether his motivation was moral or tactical, there’s a bipartisan consensus that Rubio took his role on the foreign relations committee seriously. The amount and quality of legislation, speeches and editorials he put out led some Senate staffers to privately speculate that he had a group of foreign policy heavyweights advising him behind the scenes. His rivals for the Republican nomination have criticized him for being an absentee lawmaker, especially since he began his presidential campaign, but a current Democratic aide to a member on the committee said that they almost always see one of Rubio's aides at the briefings the senator misses.

“This was more than a committee assignment for him,” said one former Senate aide who worked for another Democratic member on the foreign relations committee. “I’m sure it was part of him saying, ‘I want to run for president.’”

But even as they acknowledge the time and effort Rubio’s team put into the foreign relations committee, some who have worked with him question how hawkish he really is.

“There’s a view that he can shift in his convictions,” said the former staffer. Rubio “talked tough” on Syria’s President Bashar Assad in the lead-up to the 2013 vote and was critical of the Obama administration’s hesitance to enforce its own red line on the regime’s use of chemical weapons, the former staffer recalled. When the foreign relations committee gathered in early September to vote on a resolution to authorize the military strike, the public was wary of military action in Syria. A poll published the day before the vote showed that just 35 percent of Republicans favored a strike.

The staffer was expecting Rubio’s support. But when the committee voted, Rubio opposed the measure.

“There was no real understanding of why,” the former staffer said. Much of the speech Rubio gave on the floor after the vote sounded like a rationale to approve military intervention in Syria. “That, to me, was like, this is way too slippery here. It’s too cute by half,” the former staffer said.

At the time, Rubio claimed he “never supported the use of U.S. military force in the conflict.” But during his presidential campaign, Rubio has instead claimed that the operation described by the Obama administration was too small in scale to be effective.

And Syria isn’t the only example of an issue on which Rubio took a noticeably more hawkish position after becoming a presidential contender.  

He said in 2012 that “we should be open to negotiations with Iran,” but throughout 2015, he was an outspoken opponent of the nuclear deal being negotiated -- even before the agreement was finalized. In a fundraising appeal, he boasted that he was one of the first to sign Sen. Tom Cotton’s (R-Ark.) letter warning Iran’s political elite that a future president could unwind any deal agreed to by Obama.

In 2013, Rubio opposed reversing defense spending cuts without also cutting spending elsewhere. But his current budget plan calls for increasing the defense budget by $1 trillion over the next 10 years, with no offsetting cuts.

Rubio’s participation on the foreign relations committee dropped off after he began his presidential campaign, staffers say. And when he did make appearances at the Senate, it often seemed like it was for show. When the committee came together last year to craft a bill to allow Congress to vote on the Iran nuclear deal, Rubio had to be talked out of pushing  “poison pill” amendments that would’ve killed the bill’s chances of passing, a current committee staffer said. After agreeing to stand down for the committee vote, Rubio later tried to force a vote on the Senate floor on an amendment that would have required Iran to recognize Israel as part of the nuclear deal.

“He was auditioning for Sheldon Adelson,” said the former staffer, referring to a wealthy casino mogul who is known to fund the campaigns of politicians he believes to be the most friendly to Israel.

In July, after missing private hearings on the Iran nuclear talks, Rubio made an unusual appearance at a public hearing, where he sparred with Secretary of State John Kerry over the nuclear agreement, causing some staffers to grumble that Rubio only bothers to show up to high-profile hearings.

Rubio has kept up his focus on foreign policy in the final days before Tuesday's Florida primary, which he has repeatedly promised to win. On Friday, he met with "national security leaders" at an event in Palm Beach.

A Monday poll showed him trailing Trump in Florida -- by 24 points.