In case you missed Monday night’s Indiana Senate debate, I’ll catch you up:

Democratic Senate candidate Joe Donnelly at North Central High School in Indianapolis, where President Clinton campaigned for him on Friday.
(Melinda Henneberger — The Washington Post)

Democrat Joe Donnelly had this to say: Bipartisan, Dick Lugar, bipartisan, etc.

Republican Richard Mourdock disagreed: Principle! Debt! More debt!  

The third man on the stage, though, was not as inconsequential as he seemed, endlessly repeating that Hoosiers should break the stranglehold of the two-party system and send him, libertarian Andrew Horning, to Washington instead.

Not only because Horning got in the most exciting line of a dull, dull night — by mocking the war on terror as an effort to head off the vile threat posed by “exploding underpants.”

But also because in a race this tight — one recent poll had Donnelly up by 2, another had Mourdock up by 5 — the 5 percent of likely voters who support Horning could very well decide the outcome in Donnelly’s favor. 

As a Libertarian, he’ll draw almost exclusively from Republicans, many of whom have not forgiven Mourdock for his rough treatment of longtime senator Richard Lugar in the primary.

Richard Mourdock at a rally in Evansville last spring. (Connor Turque — The Washington Post)

And if Horning does help push Donnelly over the finish line, it won’t be the first time the congressman and former small business owner from Granger, outside South Bend, has gotten a hand from a third-party candidate; in the 2010 Congressional race he won by a smidge — 48.2 to 46.8 percent, with Libertarian Mark Vogel taking 5 percent. Though Donnelly is running on playing nice, the Democratic Party sent pro-Vogel mailers to Republicans in that 2010 campaign.

Outside money has poured into the current Indiana Senate race on both sides, and the result is the usual nonstop nasty tit for nasty tat air war. At the Indianapolis campaign event last Friday at which Bill Clinton made the case that Donnelly was a Democrat in his own likeness, I asked Donnelly at what point the mutually assured destruction of his ad war with Mourdock became a waste or worse. But his answer was about the terrible Mourdock ads; if we’d been in court of law I could’ve said, “Objection, non-responsive.

Donnelly began Monday night’s debate by describing his rival as “an unapologetic leader of the tea party movement, and that’s fine.” Not so fine with Mourdock, though, who bridles at a label that’s costing him with the state’s many Lugar supporting-moderates.

Donnelly skewered Mourdock over a fundraising letter written after the May primary that accuses the senator he’d already beaten as having “betrayed” his party.

“You can disagree, but betray?” Donnelly asked.  I don’t see how that even fits in the dialogue. We can disagree with each other, we can have different points of view, but…I don’t understand how you can say betray.”

(After the debate, Mourdock said he regretted that that letter had been sent; though sent out over his signature, it was prepared and mailed by a contractor, he said.)

During the the debate, Mourdock, the state treasurer, insisted that he didn’t mean to imply that Medicare and Social Security weren’t constitutional when he said they’re not mentioned in the U.S. Constitution.

“We’re not that dumb,” Donnelly returned. Voters “know what you were implying. And we know what you’re driving at.”

One of the biggest issues in the race is Mourdock’s attempt to sue the federal government over its bailout of the auto industry. Teacher and firefighter pension funds that Mourdock was in charge of as state treasurer had lost millions buying up Chrysler’s secured debt, and Mourdock argues that he was trying to force Chrysler’s liquidation to get that money back. Donnelly counters that if he’d succeeded, he would have lost more than 120,000 jobs in Indiana, and that Mourdock was offered a settlement for the pension funds and refused it.

Mourdock, whose assets include a demeanor as mild as Lugar’s and the habit of answering all questions directly, scored his biggest point of the night when he said that if he were the heartless hack he’d been painted as, how come one of his life’s greatest joys was working with some of the poorest people on the planet in Bolivia? Yet the Ryan budget he supports would significantly cut aid to the poor in his state.

Melinda Henneberger is a Post political writer and anchors the paper’s ‘She The People’ blog. Follow her on Twitter at @MelindaDC.