Earlier this month, Sen. Mark Kirk of Illinois became the first sitting Republican senator to withdraw his support of Donald Trump. On Thursday, Kirk took that stand to its logical conclusion: He’s now actively touting his opposition to Trump as a selling point in his re-election bid.

“Mark Kirk bucked his party to say Donald Trump is not fit to be commander in chief,” declares Kirk’s latest television commercial in Illinois. The 30-second spot is not a full-on attack ad against the GOP’s presumptive nominee, but it does amplify the Democrats’ main critique that Trump is temperamentally unfit to be president. (The ad also touts the GOP senator’s support for holding a confirmation vote on Supreme Court nominee Merrick Garland and of a “women’s right to choose” as further proof of his bipartisan bona fides.)

The commercial completes Kirk’s speedy transition from a somewhat passive Trump supporter to an active opponent of his party’s new standard-bearer. Originally, Kirk suggested he’d back the GOP nominee regardless of who it was. Later, once Trump had more or less locked things up, Kirk quibbled with a few of his positions but said he’d look past them in the name of his own re-election chances. Only after Trump threw his racist tantrum over the Mexican heritage of a U.S.-born judge earlier this month—a moment that marked the start of Trump’s current drop in the polls—did Kirk say no way. He’s since gone as far to say that Trump’s “too bigoted and racist for the Land of Lincoln.”

In other words, Kirk came to the conclusion that he was better off attacking Trump than supporting him, and this ad will make it harder for his Democratic opponent, Rep. Tammy Duckworth, to tie him to his party’s presidential nominee in their tight Senate race. The spot is also a reminder of how quickly the bottom could fall out for Trump if Republican lawmakers decide helping their nominee will hurt their own electoral chances. For Kirk’s Senate colleagues who are currently locked in their own tight re-election fights, that might be a conclusion they come to soon. If they do, that will mean a few more anti-Trump ads in key states like Florida, Ohio, and Pennsylvania.

Previously in Slate: Republicans Are Afraid Trump Will Cost Them the Senate. They Should Be.



Read more of Slate’s coverage of the 2016 campaign.