Supporters cheer as Sen Mark Kirk (R-Ill.) speaks at his primary victory party in Chicago in March. In the November general election, Kirk, a former Navy commander, faces Rep. Tammy Duckworth, a veteran who lost both legs in the Iraq war. Kirk on Tuesday dropped his support for GOP presidential nominee Donald Trump. (Kevin Tanaka/Chicago Sun-Times via AP, File) /Chicago Sun-Times via AP)

Sen. Mark Kirk, an Illinois Republican facing a difficult re-election campaign, reversed course Tuesday and withdrew his support for presumptive GOP presidential nominee Donald Trump.

Kirk is the first Republican senator up for re-election this year to break with Trump, and his decision puts new pressure on vulnerable GOP incumbents in other swing states — notably Wisconsin, Ohio, Pennsylvania and New Hampshire — to part ways with their party’s presidential standard-bearer.

About a month ago, Kirk described Trump, who won the Illinois primary, as a “net benefit” to his candidacy who was luring many new GOP voters to the polls.

But Kirk appears to have decided that Trump’s popularity with some Republican voters is not enough to stick with him through November given the businessman’s most recent controversy, surrounding his racially tinged attacks on the federal judge handling a civil lawsuit surrounding his “Trump University” real estate seminars.

In a statement, Kirk said that while he opposed the Democratic presidential nominee, “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 suffered a serious stroke in 2012 that left him reliant on a wheelchair. Last year, Trump appeared to mock a New York Times reporter for a physical disability.

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

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

Kirk is facing a strong challenge from Democratic Rep. Tammy Duckworth is a state that votes overwhelmingly for Democrats, especially in a presidential election year. Duckworth, who slammed Kirk in a Chicago speech Monday for his support of Trump, sharply responded on Twitter Tuesday to Kirk’s reversal:

Duckworth’s campaign released this statement: “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. 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.”

Still, one prominent Democrat — Sen. Jon Tester (D-Mont.), chairman of the Democratic Senatorial Campaign Committee — welcomed Kirk’s statement. “My sense is, Tammy Duckworth is a great candidate in Illinois, and by the way, I think [Kirk] did the right thing in pulling his support for Trump,” he said.

Just two weeks ago on May 20, Kirk was on the Trump train, though he said backing the businessman was akin to a “riverboat gamble,” adding: “He won the Illinois primary, in this case we have seen the Republican vote up and the Democratic vote down, so it looks like it’s a net benefit.”

But while Kirk, a former naval reserve commander, said he was comfortable running on the same ticket as the wealthy businessman, he added he disagreed with some of his ideas, including an “isolationist” foreign policy.

Kirk told senior CNN congressional producer Deidre Walsh he intends to write in former Army General David Petraeus on the ballot in November, according to a tweet from Walsh.

Kirk’s change of heart comes as Republican criticism is raging over Trump’s bashing of the federal judge, Gonzalo Curiel. Trump has complained that Curiel cannot be impartial in the case because the Indiana-born judge is of Mexican heritage, and Trump intends to build a wall along the U.S.-Mexico border.

On Tuesday morning, House Speaker Paul Ryan (R-Wis.) called Trump’s attacks on Curiel “the textbook definition of a racist comment” but would not rescind his endorsement of Trump, saying a Trump presidency would be preferable to a Hillary Clinton presidency.

[Ryan says attacks on judge fit ‘textbook definition of racist comment’]

Ryan echoed and foreshadowed the statement of many other Republicans, including Senate Majority Leader Mitch McConnell (R-Ky.), who said Trump should “quit attacking various people you competed with or various minority groups in the country and get on message.”

“I don’t think that those are appropriate comments by any stretch of the mind,” Ryan told CBS host John Dickerson in an interview taped for Sunday’s episode of Face the Nation. “Hopefully a lesson will be learned here and we can move forward with a better campaign.”

Ryan added: “When anyone in our party, least especially our nominee, run contrary to our beliefs, to our values, to our principles, we have an obligation to call them out, we have an obligation to not support those things because they don’t define who we are.”

Another GOP senator locked in a tight re-election race, Sen. Ron Johnson (R-Wis.), deflected a question on whether Kirk’s reversal would put pressure on him to make his own break from Trump.

“I know it’s generating an awful lot of questions on the part of the press,” he said.

Johnson continued: “Listen, I completely disagree with what he said on that. I’ve asked him to retract the statement, and I will continue to concentrate on the areas of agreement. Obviously I don’t agree with Hillary Clinton. … I’m running my own campaign, so I’ve got to speak to the people in Wisconsin in terms of what concerns them, and I’ll continue to do that.”

Sen. Jeff Sessions (R-Ala.), who was the first senator to endorse Trump and maintains close relations with the Trump campaign, brushed off Kirk’s withdrawal of support.

“You know, Mark’s an independent person and he does what he wants to do,” he said. “But, yeah, I would rather him be supportive.”

Asked if he was concerned that other senators might follow suit, Sessions said, “I don’t think so, but I do think it’s important that the campaign put its full focus on this campaign, the issues of the campaign, the vulnerabilities of Hillary Clinton and try to avoid distractions and issues unrelated to what the American people are really concerned about.”

Sessions asked reporters if they had seen Trump’s Tuesday afternoon news release on the Trump University case, saying he was expecting a “strong statement” from the campaign.

Told that Trump did not actually withdraw his comments or apologize, Sessions said, “I wasn’t expecting it in that form, but I’m glad it happened.”

Kirk is not the first Republican senator to withhold his support from Trump. Sen. Ben Sasse (R-Neb.) has openly entertained supporting a third-party candidate, and fellow GOP lawmakers Susan Collins (Maine), Jeff Flake (Ariz.) and Lindsey Graham (S.C.) have all refrained from endorsing Trump, suggesting they have been deeply unsettled by Trump’s beliefs and behavior.

“I’m not going around telling people: ‘Don’t endorse Trump,'” Graham said. “My view is, I can’t endorse him. I can understand wanting to support the nominee of the party. I wish I could. I just personally can’t go there. Mr. Trump does have an obligation to the people who do support to make it possible to continue supporting him.”

But Graham rejected the idea that Republicans could rescind their endorsements and deny Trump the nomination.

“I don’t see that as a viable alternate,” he said. “I don’t see a third party challenge as a viable alternative.”

David Weigel contributed to this report.