A few weeks ago, Rep. Joe Donnelly sat in the “Michigan City jail” — or at least that’s how he described the windowless call room on the first floor of Democratic National Committee headquarters.

It was a form of purgatory for Donnelly, who dialed for dollars while he waited out the bitter Indiana GOP primary between state Treasurer Richard Mourdock and longtime Sen. Dick Lugar.

Since Mourdock’s resounding victory Tuesday, Donnelly and his fellow Democrats have been bullish about their prospects for putting the Indiana Senate seat in play. But Republicans balk at the idea, pointing out that no Democrat not named Bayh has won a Senate seat in Indiana since 1970.

So just how realistic are Democrats’ chances of winning the Indiana seat? There’s a winding, narrow path to victory for Donnelly, but just about everything would have to break his way — including, to a degree, the presidential race.

“I think Joe is doing everything Joe needs to do, but some of this is out of his hands,” former Sen. Evan Bayh (D-Ind.) said in a phone interview. “If it’s a race between Joe Donnelly and Richard Mourdock, I think Joe Donnelly has an excellent chance of winning that race. If it’s a proxy for the presidential race or the dominant ideology of the two political parties, it’s harder for him to win.”

Donnelly enters the race as an underdog, even with Mourdock as his foe. But Democrats have a better shot at picking up this seat without Lugar on the ballot because they don’t have to compete with the six-term Senator’s wide crossover appeal. Accordingly, Roll Call is changing the rating of this race to Leans Republican from our previous rating of Likely Republican.

First and foremost, for Democrats to compete in the Hoosier State, national politics must play in their favor. In 2010, Sen. Dan Coats (R) walloped then-Rep. Brad Ellsworth (D) by 15 points in an open-seat race, in no small part because of the national GOP wave.

President Barack Obama doesn’t have to win Indiana again like in 2008, but he has to keep his loss under 6 points, according to a Democratic source from the state. In the meantime, Donnelly will attempt to distance himself from the president during this campaign, but his fate remains somewhat tied to the White House.

“Joe Donnelly is going to run around and try to tell everyone that he’s a different kind of Democrat, a Blue Dog, a conservative,” Indiana Republican consultant Chris Faulkner said. “But at the end of the day, it’s a team sport, and he’s wearing the wrong jersey.”

Geographically, Democrats adhere to a battle-tested equation to win statewide in Indiana, according to several interviews with operatives from both parties.

A candidate must run up the score in three reliable Democratic base areas: Lake County in Rep. Peter Visclosky’s (D) northwestern district near Chicago, Indianapolis and its suburbs in Marion County, and the “Auto Belt” around the blue-collar towns of Kokomo, Muncie and Anderson.

Secondly, Democrats must be competitive in southern Indiana in the current 8th and 9th districts. A Democrat must perform decently in areas like Vanderburgh County — the opposite side of the state from Donnelly’s home in north-central Indiana.

That’s why, for the past several months, Donnelly has spent a great deal of time in southern Indiana, near the small towns along the Ohio River. A source close to the Donnelly campaign argued that his socially conservative profile will play well in that region.

“Take the Ohio River area,” the source said. “And you take guns, [abortion] and gay rights issues off the table — we can turn back the tide a little there.”

Southern Indiana was competitive territory for Democrats until recently. The twin House seats in the southern part of the state flipped party control several times over the past decade.

But unfortunately for Donnelly, Mourdock is from Evansville, in Indiana’s southwestern tip. Also, Ellsworth was from southern Indiana (Vanderburgh), and it didn’t help him much with his Senate race.

Finally, Democrats must do a much better job than Lugar’s campaign did in painting Mourdock as an extreme, right-wing candidate. In the hours leading up to his victory, the Democratic Senatorial Campaign Committee branded Mourdock as a tea party candidate in the vein of 2010 GOP Senate nominees Sharron Angle in Nevada and Ken Buck in Colorado.

But Mourdock is more tested on the campaign trail than either of those failed nominees. Mourdock has held office for the majority of the past 20 years, including seven years as a county commissioner and two successful elections to the state treasurer’s office.

Ironically, Democrats’ greatest asset in the Senate race could be a wealth of opposition research on Mourdock that the Lugar campaign never unearthed or used. For example, Democrats regularly sent a video tracker to Mourdock events for more than  a year, while Lugar’s team only intermittently followed him during the primary.

“You’ve only seen the tip of the iceberg,” Indiana Democratic Party Chairman Dan Parker warned. “A lot of it [Lugar] couldn’t use because it was a Republican primary — either that or he didn’t do it, or didn’t find it.”