PEWAUKEE, Wis. — Senator Ron Johnson, a Tea Party conservative up for re-election in a swing state, looked out at a crowd of supporters and reassured them that he and his fellow Republicans would be resolute in blocking President Obama’s nomination of Judge Merrick B. Garland to the Supreme Court.

“We absolutely will not allow the Supreme Court to flip,” Mr. Johnson said on Wednesday evening, drawing thunderous applause at a political forum organized by a local conservative talk-radio host, Charlie Sykes, in this Milwaukee suburb.

Mr. Johnson is widely considered one of the Republican Party’s most endangered incumbents in his rematch with Russ Feingold, a liberal Democrat who served three terms in the Senate until his ouster six years ago. But as Mr. Feingold repeatedly hammers Mr. Johnson over the refusal by Senate Republicans to fill the court vacancy — an issue that Democratic leaders believe will help propel them to victory in November — Mr. Johnson is turning the tables with gusto.

“Yeah, I am hearing the drumbeat, ‘Do your job! Do your job!’ ” Mr. Johnson said, invoking the Democrats’ battle cry in their quest to get Judge Garland a hearing. “We’re doing our job,” Mr. Johnson declared, to shouts of “yes.”