With approval ratings for Congress falling somewhere below used-car salesmen and three-card monty dealers, it’s not often one will see an incumbent underperforming their individual approval rating. Yet that’s just what Jeanne Shaheen is doing in her Senate re-election bid in New Hampshire. According to a new CNN poll of likely voters, Scott Brown has pulled into a 48-all tie with Shaheen, even though she has a majority favorable rating from the same pool of voters. The difference seems to be “presidential drag”:

Scott Brown, the former senator from Massachusetts who moved to New Hampshire to run in a more friendly environment, appears to be in a dead heat with Democratic Sen. Jeanne Shaheen, a new poll shows.

A CNN/ORC International poll out Monday finds Shaheen and Brown tied among likely voters, with both obtaining the support of 48% among 735 voters surveyed. …

What could be a drag on Shaheen, however, is New Hampshire residents’ opinion of the leader of her party. Thirty-eight percent of New Hampshire adults polled approve of the job President Barack Obama is doing, while 60% disapprove. Throughout the campaign, Brown has sought to tie Shaheen to Obama.

In most calculations for a Republican majority, New Hampshire has been one of the second-tier possibilities for a flip at best. Shaheen has looked solid until now, and the carpetbagger tag on Brown has been thought to be a significant obstacle. Clearly, though, New Hampshire voters are in a forgiving mood on the latter issue. They don’t seem to be as forgiving about being a supporter of the Obama agenda, although Shaheen has to feel somewhat relieved about scoring ten points above Obama this close to the election.

An incumbent under 50% at this stage of an election is in serious trouble, especially when her opponent has already tied her in the likely-voter polls. Thanks to Obama’s unpopularity, the tilt of the turnout might go more Republican than pollsters will assume in their likely-voter screens, even in New Hampshire. A 60% disapproval rating — among all adults, it should be noted — suggests that Obama will drive turnout significantly, and not in a way that Democrats like.

John King calls this “presidential drag,” while Ron Fournier notes that Brown is surging among younger voters:

“What we’re seeing here is what we’re seeing across the country,” Fournier concludes, ” which is the President being an anvil to Democrats.” If that’s true in New Hampshire with a generally-liked incumbent, just imagine what it will look like in redder states and with less-popular incumbents in swing states.