Donald Trump's recent attack on a federal judge has multiple GOP lawmakers floating the possibility that the presidential nomination might slip away from him at the Republican convention in July.

"He's not our nominee yet," Sen. Jeff Flake, R-Ariz., said pointedly when asked about Trump and the current Supreme Court vacancy.

Trump has won a majority of GOP delegates and remains the only Republican candidate in the presidential field. But he repeatedly accused Gonzalo Curiel, a federal judge overseeing a fraud lawsuit against Trump University, of being biased because "he's a Mexican."

That has outraged and alarmed GOP lawmakers on a day when the primaries are winding down — a point in the race in which a party would typically be unifying against the Democratic nominee. House Speaker Paul Ryan described Trump's charge against the judge as the "textbook definition of a racist comment."

Most lawmakers remain resigned to the fact of Trump's victory, but they're still frustrated by his actions.

Sen. Susan Collins, R-Maine, said that Trump needs to retract the statement. "He can apologize to the judge," she said. "He can apologize to the American people to his comments."

But Trump has yet to do so, to the frustration of other Republicans. "I think his assumption is that this whole thing will blow over," another senator said. "I'm not sure it will."

On Tuesday night, Trump released a lengthy statement declaring, "I do not feel that one's heritage makes them incapable of being impartial, but, based on the rulings that I have received in the Trump University civil case, I feel justified in questioning whether I am receiving a fair trial." He didn't apologize or back down from his comments, but said he didn't intend to comment on the matter any further.

Denying the nomination to a candidate who achieved "presumptive" stature during the primaries would be unprecedented. But it could happen because the RNC Rules Committee, which meets just before the full convention, could change the rules to disadvantage Trump — perhaps by raising the the threshold for clinching the nomination on the first ballot, for instance.

"The rules committee is really not going to meet until right before the convention and they are maybe going to have a rule that is different than it is today," said another GOP senator who declined to be named. "As it is right now, I'm sure there are a lot of thoughts going on behind closed doors as to what they can do in establishing final rules for the convention."

RNC spokeswoman Lindsay Walters told the Washington Examiner that "the rules that will govern the 2016 convention will be created by the Convention Rules Committee and approved by the delegation. The RNC does not set the rules."

Flake said that is "unlikely" that the GOP would deny Trump the nomination at this stage, but he also warned that Trump is alienating Republicans. "That's certainly more likely now than it was last week," he said. "If he can't go a week without offending somebody else, what's it going to be? And there's a long time between now and November."

Sen. Mark Kirk, R-Ill., initially pledged to support the Republican nominee, but Trump's latest comments sparked a reversal. "I cannot and will not support my party's nominee for president," he said Tuesday.