WASHINGTON — Sen. Mark Kirk, R-Ill., started seriously considering revoking his support for GOP presumptive nominee Donald Trump on May 31, while visiting with workers at a Chicago factory. That was just days after Trump aimed a race-based attack at a federal judge handling two fraud lawsuits against his Trump University.

On that day, Kirk visited the Howe Corp., 1650 N. Elston Ave., and talked to some of the workers at the refrigeration equipment factory — using some of his Spanish, I’m told — and he heard their concerns about Trump.

Last Friday, Kirk listened to excerpts of Trump’s interview with CNN’s Jake Tapper, who asked Trump 23 times if his attacks against U.S. District Court Judge Gonzalo Curiel — who was born in East Chicago, Indiana, the son of Mexican immigrants — were racist.

Trump called the judge a “Mexican” who was unfair to him because he wanted to build a wall on the U.S.-Mexican border, clueless in his Tapper interview that his attacks were indeed racism.

With that, Kirk, one of the most vulnerable Republicans on the November ballot, had enough, I’m told.

Kirk decided to yank his support for Trump, which he did on Tuesday.

“I have spent my life building bridges and tearing down barriers — not building walls. That’s why I find Donald Trump’s belief that an American-born judge of Mexican descent is incapable of fairly presiding over his case is not only dead wrong, it is un-American,” Kirk said in a statement.

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Kirk said he hoped Trump would evolve, but that did not happen.

“. . . 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.

“It is absolutely essential that we are guided by a commander-in-chief with a responsible and proper temperament, discretion and judgment. 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.”

What does this mean for Kirk’s Republican support?

I don’t think he will lose a lot by cutting himself off from Trump.

There is little downside. There is upside in being the only Republican in the Senate up for re-election to reject Trump outright, since this fits into Kirk’s brand, just as when he was the first GOP senator to say President Barack Obama’s Supreme Court nominee, Merrick Garland, deserved a hearing and a vote.

Kirk can make the argument that it is as important to keep the Senate in GOP control — and that trumps Trump. In any event, there is no organized Trump organization in Illinois to object, even though he won the primary.

Kirk also has political cover. Conservatives, such as House Speaker Paul Ryan, R-Wis., also have denounced Trump, though Ryan did not pull his endorsement. On Tuesday, Ryan said Trump made the “textbook definition of a racist comment” when he castigated Curiel.

Kirk had given Trump an enormous benefit of the doubt in March, when he said he would back him if he was the Republican nominee, with the support coming just before the March 15 Illinois GOP Senate primary, and Kirk did not want to stir up conservatives who are not his fans.

On Monday, U.S. Rep. Tammy Duckworth, D-Ill., who’s battling Kirk for his Senate seat, slammed him for his silence on Trump during a speech before an EMILY’s List meeting in Chicago, a group that’s one of her biggest supporters.

“Trump’s statements are outrageous, they are un-American, and they are dangerous,” Duckworth said.

“And to my opponent, Sen. Kirk, I would simply say, as Dr. King did, there comes a time when silence is betrayal. Trump may be a clown, but he isn’t funny — he’s not a riverboat gambler to be admired.

“. . . He’s not a benefit, and any politician, regardless of party, who stays silent, or hopes to fly under the radar, is complicit in the Trump campaign of hate and division — and deserves to be judged harshly.”

On Tuesday, Duckworth, speaking to reporters in Chicago before a fundraiser for her at 510 W. Erie said of Kirk dumping Trump: “Why did it take so long?” She made it clear that Democrats will continue to link Trump and Kirk.

When it comes time to fill out his Illinois ballot, Kirk will write in the name of a presidential candidate, his campaign said.