It was sharp criticism from an incumbent who had previously insisted he would support the GOP nominee.
The Illinois Republican, who faces a tough challenge this November from Illinois Democratic Rep. Tammy Duckworth, tweeted Tuesday that based on his background in the military, he does not have faith in Trump to lead.
“Given my military experience, Donald Trump does not have the temperament to command our military or our nuclear arsenal,” Kirk wrote.
Kirk, who told CNN that he would write in former CIA director David Petraeus this fall, followed up with a statement saying he objects to Trump’s recent comments about a federal judge’s Mexican heritage.
“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.