Mark Kirk, the most vulnerable Senate Republican running for re-election in 2016, on Tuesday withdrew his support of Donald Trump’s presidential bid, citing the presumptive GOP nominee’s erratic temperament and derogatory comments toward Hispanics, women and the disabled.

The Illinois Republican is the first GOP senator seeking re-election this year to say he will not support Trump. And even as a vulnerable first-term Republican trying to win in a deeply blue state, his defection from the polarizing GOP leader is an extraordinary decision.

It also reflects how freshly nervous many Republicans are about Trump atop the GOP ticket, after he accused a federal judge of bias because of his Mexican ancestry. His comments have ignited a firestorm of criticism, including from many in Trump’s own party.

Kirk’s declaration, however, might be the boldest move yet from a Republican to distance himself from Trump. Kirk said he would not support Trump’s opponent Hillary Clinton and instead said he would write in former CIA Director and retired Gen. David Petraeus.

“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,” Kirk said in a statement issued by his campaign Tuesday.