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Mark Kirk.
Kirk made the announcement to Steve Cochran on WGN radio Thursday after making the unprecedented move to unendorse Trump last month.
"I share your concerns about Trump every day with people," Cochran said. "And then I look at the alternative choice and I never thought I'd be in a position to even consider voting for Hillary Clinton. And yet here we are with those being our two main choices. I don't know what to say to people when they say every day, 'Well, what are we supposed to do?'
"Cochran, I got a way out for you," Kirk responded. "I'm gonna, I'm gonna write in Petraeus when I vote. And I encourage you too. I want to see Petraeus get a lot of votes so that just shows that there are thinking people out there that are really concerned about the security of the United States. Thinking that someone who has an outstanding potential as commander-in-chief and can be, should be, our new leader. With these two choices that you almost think it's Tweedle-Dee dumb and Tweedle-Dee bad."
Kirk also railed on Clinton for her use of a private email system after FBI Director James Comey announced Tuesday that she was "extremely careless" with sensitive information, although he did not recommend prosecution.
Associated Press/Charles Dharapak
David Petraeus.
Petraeus, a former CIA director, disclosed classified information to his biographer. Petraeus pleaded guilty in 2015 to a misdemeanor charge of mishandling classified information.
"[Petraeus] not only shared [classified information] with someone who was not allowed to have it, but we found it in a search warrant under the insulation in his attic, and then he lied to us about it in the investigation," Comey said during a Thursday hearing before the House Oversight and Government Reform Committee.
Kirk, who is battling for his Senate seat with Democratic Rep. Tammy Duckworth of Illinois, made headlines elsewhere Thursday after he decided against attending a meeting between Trump and Senate Republican leaders.
At the meeting, Trump characterized Kirk as a loser, The Washington Post reported.
"I guess he lit me up," Kirk later told The Post.
Kirk's seat is widely considered by many to be one of the most likely to flip this fall.
Listen to Kirk's radio interview below: