A new Franklin Pierce University/Boston Herald poll shows Bernie Sanders leads Hillary Clinton 44 to 37 percent among likely Democratic primary voters in New Hampshire. In March, the same poll showed Clinton leading Sanders 44 to 8.

That is ... a big swing. Support for Clinton also looks very different from support for Sanders. The poll asked likely Democratic primary voters for their overall impressions of both candidacies. Support for Clinton's candidacy is fairly grudging, with only 35 percent of likely Democratic voters admitting to actual, authentic excitement for the Democratic frontrunner:

Sanders is just the opposite. His supporters seem thrilled by him. The main thing holding Sanders back from much bigger numbers is a fear that he's simply not electable:

But perhaps the worst number for Clinton has nothing to do with Bernie Sanders at all. The poll asked Granite Staters whether Vice President Joe Biden should run. Forty-six percent of respondents — a plurality — said he should. His favorability also increased 14 points since the March poll.

So imagine you're Joe Biden reading this poll. You want to run for president — you badly want to run for president, you've always wanted to be president — but you've just not been sure Hillary Clinton is beatable. Now you see a New Hampshire poll in which Hillary Clinton is actually, literally, losing. And losing to Bernie Sanders of all people. And in that poll, the likely Democratic voters of New Hampshire are crying out for you to enter the race. And the Democrats who most want you in the race, according to the crosstabs, are the ones who self-identify as moderate or conservative, which is to say, your likely supporters don't really overlap with Sanders's supporters.

The real problem for Clinton isn't Sanders. While it's possible he can mount a challenge in a few states, her team doesn't believe he can seriously threaten her nomination. But Biden, potentially, can. And the Clinton team's best strategy for keeping Biden out of the race is to convince him that there's no reason for him to enter the race — Clinton is an unstoppable juggernaut, and Biden will just be humiliated as he was in 2008. But if a few more polls like this one emerge, that's going to become an increasingly hard case to make.

There are two bright spots for Clinton in this poll. First, 80 percent of New Hampshire Democrats have a favorable view of her candidacy. And 65 percent believe she'll ultimately be the nominee. So her problem isn't that Democrats in the state don't like her, or even that they've lost faith in her chances. Her problem is that, as of yet, she hasn't given New Hampshire Democrats much to be excited about, whereas Sanders has.

Watch: Bernie Sanders on what scares him most about American democracy