Clinton has gone from all but ignoring Sanders to fiercely engaging him in recent days, a reflection of public and private polling that points to a race that is uncomfortably competitive for the Democratic front-runner. She openly questions his electability and argues that he is out of step with the party on guns and other issues.
But there are fresh questions about her electability argument, with Sanders faring better than Clinton in a series of hypothetical head-to-head matchups with top Republicans in Iowa and New Hampshire, according to the latest NBC/Wall Street Journal/Marist poll. Sanders holds a double-digit edge over Trump in both states, for example, while Clinton has a far narrower lead.
Still, Clinton is hoping to persuade voters who may be intrigued by Sanders' populist rage to take a longer view and embrace her candidacy, whether or not they were initially enthused.
Kathleen Jurgens, an Iowa Democrat, walked into a Clinton rally this week in Council Bluffs torn between the two. But an hour later, she said she was sold on Clinton's pitch that she was the only candidate who could stop a Republican from winning the White House.
"I really like Bernie. He's outspoken and he doesn't seem as political," Jurgens told CNN. "But at this point, you really have to look at electability and Hillary really impressed me."
The unknown question causing heartburn for Team Clinton, from its campaign headquarters in Brooklyn to offices across Iowa and New Hampshire, is how many Democrats will join Jurgens and how many will stay loyal to Sanders in a year when an anti-establishment sentiment is coursing through the electorate.
The persistent challenge from Sanders presents a critical challenge for Clinton. She's in a far stronger position in many states that hold primaries later in the nominating contest. But a loss in the Iowa caucuses on February 1 will will revive memories of Clinton's third-place finish there in 2008, which ultimately doomed her first presidential bid. She's facing an even tougher race in New Hampshire, where she won the crucial primary eight years ago.
Clinton herself penned a fundraising email on Saturday that indicated the "primary race in New Hampshire is neck and neck."
While the message was designed to lower the campaign's expectations, it also underscored the sense of worry as the voting in the opening two contests are less than a month away.
"A loss there could be a sharp blow to all the work we've done. We've got the momentum to carry this win, but our folks on the ground still need the resources to put us over the finish line," Clinton wrote. "There's a lot riding on the New Hampshire primary in just one month. It's going to be close, but I'm ready to do this."
Clinton has struggled to match the enthusiasm generated by Sanders, from building a grassroots fundraising network to drawing large crowds. She has also had trouble creating a sense of urgency among her committed supporters, which is one reason she is escalating her exchanges with Sanders.
This week, Clinton and her aides suggested Sanders was out-of-step with the Democratic Party on a host of issues. The Clinton campaign has either responded -- or attacked Sanders first -- on Wall Street reform, his ability to win in November, his support for paid family leave and gun control measures.
For months, Clinton's campaign declined to get into regular exchanges with Sanders and his aides. Clinton herself waited months to even say his name on the campaign trail. The offensive stance was on full display on Friday night, when Clinton and her aides let loose on Sanders on guns.
"When it really mattered, Senator Sanders voted with the gun lobby and I voted against the gun lobby," Clinton said, taking the unusual step of calling into MSNBC's "Hardball."
She accused Sanders of saying one thing and doing another and called on him to "introduce legislation to repeal the immunity that was given to gun makers and sellers."
Aides acknowledge Clinton has not closed the sale in Iowa or New Hampshire. The energy that propelled Sanders through the summer has not diminished, which is a potential sign of worry for Clinton. Bill Clinton is returning to New Hampshire and Chelsea Clinton is making her campaign debut there next week.
A new poll released Sunday showed a tight race in Iowa, with Clinton holding a three-point lead among likely voters over Sanders, 48-45, according to the NBC/WSJ/Marist survey
. In New Hampshire, the poll found Sanders ahead of Clinton by four points, 50-46, among likely primary voters.
"No wonder they're in attack mode," Michael Briggs, Sanders' spokesman, said after the heated week. "Secretary Clinton and her team are getting nervous and nasty because the so-called inevitable nominee anointed by the establishment eight months ago doesn't look so inevitable anymore."
While Sanders aides expected the race to intensify, the sudden escalation surprised some advisers after months of a largely timid back-and-forth, particularly compared to the raucous Republican contest.
In Iowa and New Hampshire this week, she subtly slammed Sanders on his ability to keep the White House out of Republican hands.
"Think hard about the people who are presenting themselves to you, their experience, their qualifications, their positions. And particularly for those of us who are Democrats, their electability," Clinton told Iowa voters. "And how we make sure we have a Democrat going back into that White House on January 20, 2017."
It's an open question whether Clinton's promise of electability -- reminiscent of the argument she made eight years ago against Barack Obama -- will be embraced or backfire.
The first test is February 1 at the Iowa caucuses, followed a week later in the New Hampshire primary. The length of the Democratic primary fight from there depends on the opening two acts.
CORRECTION: An earlier version of this story misstated the results of the December Des Moines Register poll.