Republicans are likely to push back hard against the attacks in coming days, and Mourdock promised Tuesday night to tie Donnelly closely to President Obama, who is unpopular in the state. But the big question of the race will likely be whether GOP-leaning independents and soft Republicans in the state decide Mourdock is simply too far right to vote for or whether he can win over enough people who fit more into Lugar's corner of the party.

A DSCC memo released Wednesday morning attacked Mourdock for backing a lawsuit against the federal government's structured-bankruptcy plan for Chrysler, saying the suit cost taxpayers $2 million and threatened more than 100,000 Indiana jobs and pointing to a Mourdock statement comparing his decision to get involved with Rosa Parks.

"By defeating Dick Lugar and nominating Richard Mourdock, the Tea Party has done it again, handing another strong pickup opportunity to Democrats," DSCC Executive Director Guy Cecil writes in the memo. "Today there are tens of thousands of disillusioned Hoosiers who have supported Dick Lugar their entire voting lives and are likely dismayed by the polarizing, extreme forces that defeated him. While Richard Mourdock is poorly positioned to appeal to these voters, Joe DonnellyJoseph (Joe) Simon DonnellyVoters Dems need aren't impressed by anti-waterboarding showboating Overnight Defense: Senate confirms Haspel as CIA chief | Trump offers Kim 'protections' if he gives up nukes | Dem amendments target Trump military parade Hillicon Valley: Lawmakers target Chinese tech giants | Dems move to save top cyber post | Trump gets a new CIA chief | Ryan delays election security briefing | Twitter CEO meets lawmakers MORE is exactly the kind of reasonable, honest, job-focused centrist these voters have always supported."

American Bridge, a Democrat-affiliated outside group, sounded a similar note Tuesday night. "The Republican primary has been a referendum on Lugar, not an endorsement of Richard Mourdock," the group wrote in a memo. "As Hoosiers across the state learn more about Mourdock, they will realize that his soft-spoken demeanor belies a fervent, extreme view of politics and governance that would hurt Indiana's middle-class families, students, veterans, women, seniors and farmers."