Republicans see Indiana Sen. Joe Donnelly as the most vulnerable Democrat on a Senate map that heavily favors the majority party, but a divisive primary filled with bitter and personal attacks has left the GOP candidates bruised and given Democrats hope they can protect their endangered incumbent.

Donnelly’s path to re-election is perilous in a state that President Trump won by nearly 20 percentage points, a massive swing since President Obama won it narrowly in 2008. Adding to that fundamental challenge is the “Accidental Senator” moniker that Republicans have attached to Donnelly, referring to his first election to the chamber six years ago when he ran against a flawed candidate whose self-defeating comments on rape stirred national controversy.

Democrats hope that the acrimony damages Republicans beyond the May 8 primary, forcing the eventual nominee to spend time and resources mending the fractured base. Donnelly, meanwhile, has spent months attempting to beef up his bipartisan credentials and focus on centrist issues. Indiana Democratic Chairman John Zody called it a “jump-start” for the incumbent.

“We’ve already worked it out and we’re going full steam ahead,” he said.

Republicans acknowledge the primary race between Reps. Luke Messer and Todd Rokita and businessman Mike Braun has been perhaps the most negative of this election cycle, and don’t dispute that it has aided Donnelly. Additionally, they don’t know who will emerge as the nominee, as there has been a dearth of public polling on the contest. But strategists and party officials in Washington and in the Hoosier State insist that once the dust settles, any of the three would be formidable against Donnelly.

That puts Indiana in a separate category from other intraparty contests, including Arizona and West Virginia, where establishment Republicans have made their preferences clear regarding certain candidates they believe would wilt against Democratic opponents and sacrifice otherwise winnable seats for the party.

Still, Democrats believe the Hoosier Republican candidates’ vulnerabilities have been packaged together, allowing them to start attacking immediately after the primary. All three challengers have had negative stories in the press that Democrats could highlight in the general election campaign, and they’ve been running negative ads attacking one another on TV. In a debate earlier this month, Braun -- a former state legislator -- said his two opponents had made politics their career; Rokita argued he was the only Republican on stage who hadn’t voted to raise Hoosiers’ taxes; and Messer called Braun a “lifelong Democrat” and said Rokita “doesn’t vote with the president on the tough votes.”

One veteran Indiana Republican strategist admitted that “no question, Donnelly is happy to have” such enmity expressed among his rivals, but added, “I really don’t think there’s a long-term effect that’s devastating for Republicans here. These guys are all candidates who can win in the fall. They don’t give me any kind of heartburn.”

Some Democrats privately agree that the primary acrimony itself isn’t enough to swing the race in Donnelly’s favor. One prominent party member involved in previous Senate races, who requested anonymity to speak candidly, said Donnelly needed to improve his name ID in the state, and said he faces an “uphill climb.”

“I do think that the immature squabbling … has hurt those three candidates,” the Indiana Democrat said. “I don’t know if it’s enough to overcome the Republican domination in the state.”

Republicans are determined to avoid mistakes that cost them precious votes in the last race for this seat. In 2012, Tea Party candidate Richard Mourdock shocked longtime Sen. Richard Lugar in the primary while Donnelly earned the Democratic nomination unopposed. Republicans worked feverishly to repair the damage and coalesce behind Mourdock, but his campaign collapsed after he made comments about rape and pregnancy during a late-October debate. Donnelly won the race by five percentage points, while President Obama lost the state by 10.

“Folks remember that and say, ‘This is our chance to right that wrong and put in a conservative Republican,’” one GOP state operative said.

Democrats bristle at the notion that Donnelly was simply lucky six years ago, and say the senator is a hard worker who has an everyman appeal in the state. He has voted with Trump more than almost any other Democrat, including to confirm Supreme Court Justice Neil Gorsuch and Secretary of State Mike Pompeo. His first campaign ad featured Donnelly driving an RV and saying that to “navigate that mess in Washington ... you’ve got to be willing to drive down the Hoosier common-sense middle.”

“The Republican base in Indiana is substantial, the Democratic base less so,” noted one Democratic Senate operative, who argued Donnelly would need to win most Democrats while splitting independent voters and picking up a chunk of moderate Republicans. “Fortunately this primary has been helping that along.”

Donnelly has also posted solid fundraising numbers, holding $6.4 million in his campaign account and raising significantly more than Messer and Rokita in the first months of this year. Braun, however, has primarily self-funded his campaign during the primary and could lend himself significant additional resources to hit the ground running in the general election.

National Republicans are likely to spend heavily against the incumbent, labeling him as an opponent of Trump. Already, Americans for Prosperity, a group backed by the Koch brothers, has spent several million dollars attacking Donnelly for opposing Republicans’ tax cuts. Additionally, Vice President Mike Pence, the former governor of Indiana, will likely play a key role in turning voters out for the GOP nominee and tying Donnelly to the national Democratic Party.

“A lot of people don't realize he's a Democrat. Some folks just assume because he's a statewide official that he's a Republican,” said a state GOP operative. “So job number one is making sure people know he's a Democrat.”

James Arkin is a congressional reporter for RealClearPolitics. He can be reached at jarkin@realclearpolitics.com. Follow him on Twitter @JamesArkin.