Sen. Mark Kirk Mark Steven KirkLiberal veterans group urges Biden to name Duckworth VP On the Trail: Senate GOP hopefuls tie themselves to Trump Biden campaign releases video to explain 'what really happened in Ukraine' MORE (R-Ill.) on Thursday slammed presumptive GOP nominee Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE, saying the White House candidate is "too bigoted" for the state of Illinois.

“I do not support Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE and I told the public that I did not support Donald Trump, either. I think he’s too bigoted and racist for the Land of Lincoln," Kirk, a vulnerable senator seeking reelection, said during an interview with "Big" John Howell on Chicago's WLS-AM 890 first reported by Buzzfeed.

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Kirk was asked whether he thinks Trump or presumptive Democratic nominee Hillary Clinton would be a better commander in chief.

The Illinois senator said instead of voting for either Trump or Clinton, he plans to write in former CIA Director David Petraeus.

"I think if he announced, he would just light up the political spectrum," Kirk said.

Earlier this month, Kirk said he would not vote for Trump. He said he found the billionaire's comments that an Indiana-born federal judge may be biased against him because of the judge's Mexican heritage "dead wrong" and "un-American."

“As the presidential campaign progressed, I was hoping the rhetoric would tone down and reflect a campaign that was inclusive, thoughtful and principled,” Kirk had said in a statement.

“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.”

In the statement, Kirk said Trump does not have the "temperament necessary to assume the greatest office in the world."

The campaign for Rep. Tammy Duckworth (Ill.), the Democrat who is challenging Kirk, criticized Kirk's initial support for Trump.

“It was already perfectly clear who Donald Trump was when Mark Kirk endorsed him: a demagogue with a penchant for racist and sexist attacks on other Americans. While Trump has long demonstrated that he lacks the judgment, temperament, or even the values to be President of the United States, Kirk not only chose to endorse him, he looked forward to Trump being a ‘net benefit’ to his campaign," said Duckworth campaign spokesman Matt McGrath.

"Now that Trump’s increasingly unhinged campaign looks like an anchor instead of a blessing, Kirk is moving on. The question Kirk needs to answer, and which Illinois voters will certainly wonder, is why he ever supported Trump in the first place."

--This report was updated at 1:51 p.m.