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(CNN) —  

Mark Kirk, one of the most vulnerable Senate Republicans in the country, is betting that he can survive a tough election season by taking on Donald Trump.

The Illinois senator became the first Republican last month to release an anti-Trump television ad, which said his party’s presumptive nominee “is not fit to be commander in chief.”

The 32-second spot, running in the Chicago market, outlines Kirk’s efforts to present himself to voters in his liberal-leaning state as more moderate than his fellow Republicans in Washington.

“Mark was the first Republican to support a vote on President Obama’s Supreme Court nominee,” the television commercial says.

Kirk is one of a handful of endangered Republican senators up for re-election this November who are grappling with Trump’s controversial candidacy. At times, Kirk seems more focused on Trump than his Democratic opponent, Rep. Tammy Duckworth, going so far as reversing his support for Trump last month. Kirk’s success could determine whether the GOP keeps its majority status in the Senate.

Kirk’s opposition to Trump kicked into high gear last month amid the controversy over the presumptive GOP nominee’s racially charged criticism of a federal judge.

“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 at the time.

Trump later issued a statement saying that his comments were “misconstrued” by the media.

A few weeks into his dump Trump strategy, Kirk says bucking the top of his ticket is paying off.

“It’s helped because I’m a very independent voice for Illinois,” he told CNN, adding that his strategy helps “make sure that people know that I always put Illinois first.”

A Kirk campaign official tells CNN that there has been an uptick in donor and volunteer interest since the senator distanced himself from Trump.

His Democratic opponent Rep. Tammy Duckworth, who was quick to capitalize on Kirk’s about face on Trump, maintains that Kirk still equals Trump.

“Mark Kirk is (using) the exact kind of rhetoric as Donald Trump and he just needs to own it and stop lying about it,” Duckworth told CNN.

When asked if he could win in November without supporting the top of his ticket, Kirk told CNN, “I have always run way ahead of my ticket.”