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 (Ill.) on Tuesday said he will not vote for 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 for president.

Kirk said he found Trump's remarks about Indiana-born federal judge Gonzalo Curiel, who is overseeing a lawsuit against Trump University, "dead wrong" and "un-American."

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"As the presidential campaign progressed, I was hoping the rhetoric would tone down and reflect a campaign that was inclusive, thoughtful and principled," Kirk, perhaps the most endangered Republican in the Senate, 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," he said.

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

Trump has been under fire for saying Curiel was biased because of his Mexican heritage.

Kirk is seeking reelection in a state where 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 will be the runaway favorite in the presidential election. President Obama won Illinois by large margins in 2008 and 2012, and it is seen as a safe Demcoratic state in presidential contests.

Kirk, who will face Rep. Tammy Duckworth (D-Ill.) this fall, had previously said he’d support the eventual Republican nominee.