WASHINGTON — In sending the president a joint resolution condemning "racist violence" in Charlottesville, Congress gave President Trump a choice: sign the resolution and reject white supremacists, or veto it and align with the far right.

Trump chose a third option: Sign it — but with a signing statement attached.

The presidential signing statement has long been a controversial presidential tool that allows presidents to sign bills even as they attempt to reinterpret them. But Trump's use of a signing statement on a non-binding sense-of-Congress resolution may break new ground, experts say.

"This is extraordinary," said Christopher Kelley, a Miami University political scientist who has studied presidential signing statements. "It is one of the weirdest, rarest uses of a signing statement that I know of."

The resolution, passed by voice vote this week by the House and Senate, urged the president to specifically "speak out against hate groups that espouse racism, extremism, xenophobia, anti-Semitism, and white supremacy." It was a response to Trump's comments on a the "Unite the Right" march on Charlottesville last month in defense of Confederate statues, in which Trump appeared to cast equal blame for the violence on left-wing protesters. 

And it specifically blamed "torch-bearing White nationalists, White supremacists, Klansmen, and neo-Nazis" for the violence that killed a protester and put two state troopers in harm's way in a fatal helicopter crash.

Trump signed the resolution late Thursday. His signing statement said that Americans "oppose hatred, bigotry, and racism in all forms."

"No matter the color of our skin or our ethnic heritage, we all live under the same laws, we all salute the same great flag, and we are all made by the same almighty God," Trump said. "We are nation founded on the truth that all of us are created equal."

But just as his initial statements on Charlottesville blamed "all sides" for the violence, Trump's signing statement didn't condemn any specific group. 

"It is ironic that this bill is in response to Trump’s tone-deaf comments about 'all sides' and then when he signs this statement, he repeats the very thing that sparked the controversy in the first place," Kelley said.

Indeed, some members of Congress suggested that the signing statement — combined with Trump's renewed remarks Thursday holding the left-wing "antifa" protesters equally responsible for the violence in Charlottesville — showed a lack of sincerity by the president. 

More: Trump signs resolution condemning 'racist violence' in Charlottesville

Sen. Bob Casey, D-Pa., tweeted that Trump's "refusal to hold white supremacists fully accountable for Charlottesville continues to be an insult to our nation."

Sen. Mark Warner, D-Va., the sponsor of the resolution, said he was pleased that the president signed it, but that "unfortunately he still equivocates when he speaks," he tweeted.

"In what way did he equivocate?" White House press secretary Sarah Sanders responded.

The resolution expresses a sense of Congress and forms part of what's known as "soft law" — a form of legislation that has no enforceable provisions. The Charlottesville resolution contains verbs like "condemns," "recognizes" and "expresses" and "rejects."

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Congress could have adopted it by concurrent resolution, which wouldn't have required the president's signature. By passing a joint resolution, it forced him to take a stand. 

"I do think that was the point: to force him either to repudiate his earlier statements by signing or to double down. The signing statement was an effort to avoid these two outcomes," said Eric Posner, a University of Chicago law professor who has studied the use of soft law. "This is all a matter of political rhetoric and sending signals to supporters."

But beyond encouraging the president to use all available resources to combat hate groups, the new law has no legal significance. 

"The fact that the president used a signing statement to convey his views does not mean anything beyond what he says in it," Posner said. "The resolution has no legal effect and the signing statement has no legal effect."