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Presumptive Republican nominee Donald Trump has received his first un-endorsement as the firestorm expands over his racially-focused attacks on a federal judge.

Sen. Mark Kirk, Republican of Illinois, released a statement Tuesday saying that Trump lacks "the temperament" necessary for the Oval Office.

"After much consideration, I have concluded that Donald Trump has not demonstrated the temperament necessary to assume the greatest office in the world," Kirk said in a statement.

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Kirk, who won President Barack Obama's seat, is running for re-election in a Democratic state in what is considered one of the most contested races in the country.

His opponent, Democratic Rep. Tammy Duckworth, had been criticizing Kirk for his silence over Trump's comments.

"What took so long?" Duckworth said in a statement.

Before Trump earned the nomination, Kirk said that he "certainly would" support him if he won the nomination.

But Trump's latest attacks against Judge Gonzalo Curiel, who Trump said won't give him a fair trial on a Trump University suit because he's "Mexican," is a bridge too far, Kirk says.

"As the Presidential campaign progressed, I was hoping the rhetoric would tone down and reflect a campaign that was inclusive, thoughtful and principled. 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 cannnot and will not support my party's nominee for President regardless of the political impact on my candidacy or the Republican Party."

While Kirk is the first candidate to take back his support, he is one of more than a dozen Republican leaders who have denounced Trump's latest comments against Curiel.

Related: Paul Ryan: Trump's Comments Are 'Indefensible'

On Tuesday alone, the top Republican in Congress had harsh words for Trump. House Speaker Paul Ryan called them "the textbook definition of a racist comment” and Senate Majority Leader Mitch McConnell advised Trump to "stop attacking ... various minority groups ... and get on message."