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Indiana's open-seat Senate race remains a dead heat, with Democratic Rep. Joe Donnelly running neck-and-neck with state Treasurer Richard Mourdock, according to a new bipartisan poll released Thursday.

Donnelly leads Mourdock in the poll, conducted for the subscription newsletter Howey Politics Indiana and DePauw University, 40 percent to 38 percent. That is well within the poll's margin of error of plus-or-minus 3.5 percentage points. Libertarian candidate Andrew Horning siphons 7 percent of the vote, the poll shows. The poll was conducted by Fred Yang of the Democratic firm Garin-Hart-Yang Research Group and Christine Matthews, president of GOP polling firm Bellwether Research.

Yang writes for Howey Politics Indiana that while the race is a virtual tie, Donnelly has a few built-in advantages. He has a more positive image rating (24 percent favorable/21 percent unfavorable) than Mourdock (26 percent favorable/32 percent unfavorable). That is particularly true among independent and undecided voters, Yang writes, though many of them "have yet to get a firm grasp of either candidate."

Mourdock has struggled to unite his party after his victory over long-time Sen. Richard Lugar in a divisive primary. Just 60 percent of Lugar supporters say they will vote for Mourdock in the general election; by comparison, 86 percent of Lugar supporters say they will vote for the GOP's presidential ticket. But Donnelly has work to do to persuade Lugar's voters to break with their party: He wins only 15 percent of them, just a tick higher than his party's presidential ticket.

Overall, Mitt Romney leads President Obama in the state, 52 percent to 40 percent; Hoosier voters narrowly awarded their state's 11 electoral votes to Obama in 2008. That suggests the environment tilts toward Mourdock, though he is not yet able to capitalize on it, winning just 71 percent of Republican voters. Donnelly, on the other hand, earns 78 percent of the Democratic vote.