DSCC ad labels Heitkamp independent

"Rick Berg votes the way he does because Washington tells him what to do," LaDuke says.

She cites his votes in favor of Rep. Paul Ryan's (R-Wis.) budget, which the ad says replaces Medicare with a voucher program — a common refrain from Democrats.

In contrast, the ad continues, Heitkamp is "independent" and "will stand up to Washington." And that message, if Democrats can make it stick, could be a winning one for Heitkamp — she needs to convince voters in a red-leaning state that she won't vote the Democratic party line.

But that's just what the Berg campaign hopes voters will think about Heitkamp. In an attempt to tie the candidate to the president, it issued an ad on Tuesday highlighting an interview from the 2008 Democratic National Convention during which Heitkamp said that she thinks "Barack Obama's gonna be amazing."

Though Berg has led in most of the recent polls, and Sen. John McCain (R-Ariz.) won the state in 2008, Democrats see this as one of their best opportunities to hold on to a seat, and the Democratic Senatorial Campaign Committee has been pumping funds into the race for some time now to ensure that happens. But Republicans will fight fiercely to take one of the seats that they see as key to gaining control of the Senate.