Since her official announcement, Hillary Clinton has been the frontrunner in the Democratic race for president. Perhaps she still is, but the dynamics are changing, and they're favoring Bernie Sanders. Most pundits – myself included – have found it difficult to see a clear path to the nomination for Sanders: Clinton has the support of the Democratic establishment and an appreciable organizational advantage over Sanders, and that counts for a lot in our system.

But Sanders has run a smart and aggressive populist campaign, generating a groundswell of grassroots support within the party. He's shattered records for individual contributions and he's energized a restless base with an authentic progressive message.

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Now it appears his strategy is paying off.

Two polls conducted by NBC and Marist College in Iowa and New Hampshire were released on Sunday, and the results show Sanders and Clinton neck and neck in the first two primary states. Among likely Democratic caucusgoers in Iowa, Clinton leads Sanders 48 percent to 45 percent, with O'Malley registering at 5 percent. In New Hampshire, Sanders leads Clinton 50 percent to 46 percent (O'Malley polled at 1 percent).

The Vermont Senator was expected to do well in New Hampshire (for obvious reasons), but a win in Iowa, where Clinton has committed considerable resources, would be a major coup. Clinton can absorb a loss in New Hampshire and still coast to the nomination, but an early defeat in Iowa could change the entire narrative. Momentum is crucial in a campaign, and it can shift in a hurry. Early primary states can alter the trajectory of race, particularly if it's an insurgent candidate like Sanders, who is running against the Democratic establishment as much as he is Clinton. But if Sanders manages to sweep Iowa and New Hampshire, which is now a distinct possibility, the race will be wide open.

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The plausibility arguments against Sanders have been losing their steam for months, and they're getting less plausible by the day. Still, though, Clinton is well-positioned beyond Iowa and New Hampshire. She leads comfortably in the next two primary states, South Carolina and Nevada, and, as Chris Cillizza pointed out, she is in great shape "because the rest of the calendar is made up of much more racially diverse states - Nevada on Feb. 20, South Carolina on Feb. 27 and so on and so forth - where Clinton runs far better than Sanders."

But those numbers could shift quickly, especially if Sanders takes Iowa and New Hampshire. There's been an aura of inevitability around Clinton for the majority of this campaign. Democrats tacitly accepted that she would be the nominee. But the more credible Sanders becomes as a candidate, the more likely primary voters are to give him a second look, as they appear to be doing already.

Even more problematic for Clinton is the fact that many voters have yet to pay serious attention to the race. Many people won't tune in until the primary season begins, and if Sanders and Clinton are running neck and neck, it's all up for grabs. When you combine this with the emerging head-to-head surveys of Democratic candidates against likely Republican nominees, the landscape looks increasingly favorable for Sanders.

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As Sanders argued last week on ABC's “Good Morning America,” not only is he competitive against Clinton among Democrats, he's also doing comparatively well against Republican frontrunners in national surveys: “Well, I would suggest that Secretary Clinton look at the last Quinnipiac poll, which has me leading Trump by a significantly higher margin than she does, and that's true of other polls as well...I think we can do better against Trump and other Republicans than can Secretary Clinton.”

All of the relevant numbers are trending in Sanders direction right now. That could change, of course, and Clinton remains the most likely Democratic nominee. But make no mistake: This is a race. And if Team Clinton doesn't know it yet, they're in trouble.