As the bruising Republican primary fight moves south, the rivalry between Jeb Bush and Marco Rubio is nearing a turning point.

Rubio’s advisers see South Carolina as something close to make-or-break after a disastrous fifth-place showing in the New Hampshire deeply wounded his candidacy. Bush, whose campaign has found new life in recent days, is trying to notch his second consecutive victory over Rubio – something his team hopes would sideline his rival as competition in the center-right lane.

Indeed, while Donald Trump and Ted Cruz duel for first, the battle between these Floridians has become a primary of its own, with the loser likely to face significant pressure from donors to plot an exit from the race, according to sources close to both men.

“It’s a cage match. One man wins, one man leaves. This is a political reality,” said South Carolina Rep. Mark Sanford, a former two-term Republican governor. “Any comparisons and contrasts will be made.”

This drama lands center stage at Saturday night’s debate here – a crucial showdown one week after Rubio’s widely panned performance in the pre-New Hampshire forum and a week before South Carolina votes.

Rubio spent part of Friday and Saturday holed up with advisers in prep sessions. He’s been using the trusted circle of advisers that he’s relied on since the race’s outset, including strategists Todd Harris, Terry Sullivan, and Heath Thompson, two sources close to Rubio said. On Friday, Lanhee Chen, a former top policy adviser to Mitt Romney who also helping with debate prep, was spotted in the Marriott hotel where Rubio is staying.

The senator has vowed to be more aggressive on the debate stage and has taken responsibility for his poor performance last week. In interviews this week, he conceded that he made a mistake by not fighting back more forcefully against Chris Christie, who cast him as overly ambitious and unready to occupy the Oval Office.

Rubio also telegraphed what many expect to be his central charge against Bush – that, contrary to how the former Florida governor has sold himself, he lacks the experience to be president.

“Jeb — he doesn't have any foreign policy experience. He has none,” Rubio told reporters during a Thursday breakfast. “And the most important job of the president is to be a Commander in Chief.”

At a pre-debate rally on Friday night, Rubio criticized Bush for his support of Common Core education standards.

Mike DuHaime, a top political adviser to Christie, who dropped out of the race following a poor performance in New Hampshire, said the pressure was on Rubio Saturday night. But there was a flipside, he argued. With the attention on Rubio, a good night meant that he could reach voters and position himself for a rebound.

“It makes it a great opportunity,” DuHaime said.

The South Carolina battle follows months of slow-building tensions between the two pols. Among Bush’s advisers, there is a visceral sense that Rubio, who was a Bush lieutenant while serving in the Florida legislature, has been disloyal. On conference calls with top Bush donors, one fundraiser for the campaign said, the senator has been called “Judas.”

South Carolina is known for its bareknuckle brand of politics and many are convinced that the Bush-Rubio showdown will turn negative. Both campaigns are led by aggressive tacticians – for Rubio, Sullivan, a veteran operative who spent years waging campaigns in the South Carolina trenches. Overseeing the Bush effort, meanwhile, is Danny Diaz, a hard-charging strategist who specializes in the political art of opposition research.

On Thursday, Diaz sent an internal memo to campaign staff that accused the Rubio team of playing dirty. He did single out any of Rubio’s staffers specifically, but he pointed to a recent mailer that a pro-Rubio super PAC released last week that invoked Bush’s mother, former first lady Barbara Bush. The mailer showed a doctored photograph of Jeb Bush among a lineup of royals: Queen Elizabeth, Henry VIII and the king from Burger King. Underneath the images ran a quotation from Barbara Bush: “We’ve had enough Bushes.”

“We understand due to the senator’s performance in the most recent debate and fifth place finish in New Hampshire, his team feels like it has something to prove and seeks to further a reputation for employing questionable tactics in South Carolina,” Diaz wrote.

A Bush spokesman, Tim Miller, dismissed talk of personal animus between the two men. "There are always a lot of people who like to create drama and blow up perceived slights into these huge deals."

For Bush, however, success in South Carolina will not simply depend on his ability to stop Rubio. He also must beat back John Kasich, who notched a strong second-place showing in New Hampshire and who is the third candidate competing for support in the establishment lane. While Kasich is unlikely to fare well in the state, some Bush aides are concerned that the moderate Ohio governor might siphon off votes that would otherwise go to their candidate. Right to Rise, a pro-Bush super PAC, is set to begin airing a TV commercial with images of ISIS fighters that calls Kasich “dangerously wrong” on national security – a key issue in a state rich with military interests.

Bush is looking to reinforce his national security credentials by bringing his brother, former President George W. Bush, onto the campaign trail with him. The two are slated to appear on Monday in North Charleston.

Not everyone sees South Carolina as do or die for Bush and Rubio, however. With Nevada holding its primary three days later, on Feb. 23, and a large slate of southern states voting on March 1, some see the two fighting it out for weeks to come – regardless of who beats out whom in South Carolina. They may even go head-to-head in a home state dual in Florida, which holds its primary on March 15.

“Florida will be the real climax and the state that truly determines which of them continues in the race,” said Christian Ferry, who served as campaign manager for Lindsey Graham’s presidential bid. “South Carolina is another battle in the war, but I think it is far from anyone's Waterloo.”

It’s clear, though, that Rubio recognizes the importance of South Carolina.

On Friday morning, Rubio, wearing gym shorts, came down to the lobby with his family. After buying coffee and breakfast, they gathered around a TV airing Fox News coverage of the campaign as well as ads both supporting and bashing Rubio. 

"We obviously need to do better than we did in New Hampshire," he told reporters aboard his plane on Wednesday morning.