U.S. Senate candidate, Rep. Joe Donnelly (D-Ind.) (Courtesy of CQ-Roll Call, INc.)

And if Mourdock does win tonight, those outside conservative groups that boosted his candidacy have handed Democrats an opening.

The polling has long shown that moderate Democratic candidate Joe Donnelly, a former small business owner and House member from the South Bend area, had no shot against Lugar but does stand a chance against Mourdock, a tea party favorite with big-dollar support from the Club for Growth, the National Rifle Association and FreedomWorks.

When I interviewed Donnelly in Indianapolis several weeks ago, he insisted that he was just going about his own campaign business, thanks — talking jobs, jobs, jobs all over the state — and not worrying about which candidate the Republicans settled on.

But if Mourdock wins, Donnelly does, too, for now anyway, and begins the general election contest up slightly over Mourdock in polls.

A pro-gun, pro-life, pro-Keystone pipeline, anti-illegal immigration and anti-climate change legislation Blue Dog, Donnelly has a lot in common with Lugar, especially as someone who sees working across the aisle as the only way to get anything done: “I hear from everybody, and they say ‘Joe, nowhere but in Washington do they think not working together makes sense.’ We’re not hired to fight. My question about everything I do is, does it make our country stronger?’’

The longtime owner of a small printing business, Donnelly ran ads distancing himself from House Speaker Nancy Pelosi in ’10 – and even won the NRA endorsement over his conservative Republican opponent, Jackie Walorski, who is running again this year.

So how will he run against Mourdock? Here’s one way: The RV industry that’s a major employer in his Northern Indiana district would have lost its suppliers and been put out of business, he says, if Mourdock’s suit challenging the Chrysler bailout had been successful.

“Mourdock hired the attorneys who would have forced liquidation and led to a Depression in our state. This core industry would have gone away.’’

He also parts company with the tea party’s candidate on the role of government, which he feels should be as small as possible while still enforcing regulations of the sort that would have prevented the financial meltdown in the first place. “It’s a lot like being an umpire,’’ he says. ”You hope to be as little part of the conversation as possible.’’

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