Mark Kirk, the most vulnerable Senate Republican running for re-election in 2016, on Tuesday withdrew his support of Donald Trump’s presidential bid, citing the presumptive GOP nominee’s erratic temperament and derogatory comments toward Hispanics, women and the disabled.    

The Illinois Republican is the first GOP senator seeking re-election this year to say he will not support Trump. And even as a vulnerable first-term Republican trying to win in a deeply blue state, his defection from the polarizing GOP leader is an extraordinary decision.  

It also reflects how freshly nervous many Republicans are about Trump atop the GOP ticket, after he accused a federal judge of bias because of his Mexican ancestry. His comments have ignited a firestorm of criticism, including from many in Trump’s own party.  

Kirk’s declaration, however, might be the boldest move yet from a Republican to distance himself from Trump. Kirk said he would not support Trump’s opponent Hillary Clinton and instead said he would write in former CIA Director and retired Gen. David Petraeus.   

“While I oppose the Democratic nominee, Donald Trump’s latest statements, in context with past attacks on Hispanics, women and the disabled like me, make it certain that I cannot and will not support my party’s nominee for president regardless of the political impact on my candidacy or the Republican Party,” Kirk said in a statement issued by his campaign Tuesday.      

Kirk uses a wheelchair after suffering a stroke in 2012.  

The former House member from the Chicago area also accused Trump of lacking the proper temperament to be commander in chief.  

“It is absolutely essential that we are guided by a commander in chief with a responsible and proper temperament, discretion and judgment,” he said. “Our president must be fit to command the most powerful military the world has ever seen, including an arsenal of thousands of nuclear weapons.”  

Kirk added: “After much consideration, I have concluded that Donald Trump has not demonstrated the temperament necessary to assume the greatest office in the world.”  

For months, Kirk had insisted that he would still support Trump as the nominee despite his history of controversial rhetoric. In May, he told CNN  that the New York billionaire’s presence in the race might be helpful to his own re-election effort.  

But a source inside Kirk’s campaign said the senator had hoped becoming the GOP nominee would soften Trump’s rhetoric. When it didn’t, he decided to withdraw his support.  

“Kirk really had hoped the campaign would change its tone and tenor,” said the campaign official. “But it just got progressively worse.  Mark is a guy who broke from his party on marriage equality, supports comprehensive immigration reform and supported a hearing for (Supreme Court nominee Merrick) Garland. The campaign was just not reflecting the inclusiveness needed for Republicans to be successful.”  

Kirk’s opponent, Rep. Tammy Duckworth, D-Ill., has tried tying him  to Trump. In a statement Tuesday, an official with her campaign said that it shouldn’t have taken this long for Kirk to disavow his party’s presidential nominee.  

“What took so long? Apparently for Mark Kirk, it’s acceptable to refer to Mexicans as rapists; to propose banning Muslims from entering the country; to call women fat pigs and dogs; to mock a reporter’s disability; and to insult just about everyone who doesn’t look like Donald Trump,” said Matt McGrath, Duckworth’s deputy campaign manager. “Until today, and for nearly a year, Kirk was fine with all of that, and even saw a ‘net benefit’ in Trump’s campaign, and offered himself up as a potential adviser.”  

Democrats consider the Illinois race a must-win to retake a Senate majority. Even before Trump emerged as the party’s presumptive nominee, they were confident of defeating a lawmaker who won this heavily liberal state in 2010 over a scandal-tarred Democratic nominee amid the Republican wave election.  

Kirk has also hurt his candidacy with a string of gaffes that had even allies worrying about the fallout, including suggesting that President Barack Obama wanted to give Iran nuclear weapons and calling Sen. Lindsey Graham a “bro with no ho.”    

But Kirk also successfully held a left-leaning Chicago-area House seat for a decade. And his moderate profile — he has supported gay marriage for years and backs abortion rights — appeals to the swaths of moderate voters in and near Cook Country who often decide statewide elections in Illinois.  

Kirk’s decision to defect from Trump stands out from other Republicans running for re-election this year, who have criticized Trump’s comments but said they still support his bid for the White House. New Hampshire Sen. Kelly Ayotte, for example, has called Trump’s comments about federal Judge Gonzalo Curiel “offensive and wrong” but declined to withdraw her support.  

Republican candidates have been hesitant to distance themselves from Trump because for all his deep unpopularity among many moderate voters, he commands a near-fanatical following among many Republicans and working-class white voters. Criticize Trump, many Republicans fear, and they’ll alienate a voter bloc they still need come Election Day.  

Bridget Bowman contributed to this report.

Get breaking news alerts and more from Roll Call on your iPhone or your Android.