That is, he's retiring from the U.S. Senate. He is not, however, of a retiring disposition. He's been quite forward now that he has nothing to lose, and is standing up for fiscal conservatism, to the dismay of both parties.
Harry Reid tried tried to push the annual "Doc Fix" (every year we extend this "temporary" measure) and a bunch of highway funds and unemployment benefits through at the last minute. Bunning demands the $10 billion be paid for, presumably in cuts elsewhere. A worthy cause - but very few Republicans are rallying to his side.
This is a 'Come to Jesus' moment for fiscal conservatives: is it more important to keep highway money flowing or to force some austerity on Washington's credit card spree? If budget hawks stand together here and say, "Yes, it's a worthy bill, but it must be paid for", I believe that American voters will understand. Sure, highway contractors will be ticked off, and some unemployed people might have to scramble and look for jobs a lot harder, but the votes you lose among the special interests will be more than compensated by the votes you gain in supporting the general interest of American taxpayers.