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WASHINGTON — Former Vice President Joe Biden remains the frontrunner in the race for the Democratic nomination, while Massachusetts Sen. Elizabeth Warren has returned to the level of support that preceded her autumn surge, a new NBC News/Wall Street Journal poll shows.

Biden gets the support of 28 percent of Democratic primary voters, statistically unchanged from his standing in the NBC/WSJ October poll, while Vermont Sen. Bernie Sanders stands at 21 percent and Warren has 18 percent.

Warren’s 18 percent share is a 5-point drop from her level of support in October and a 7 point fall from her peak in September.

The trio of top candidates is trailed by South Bend, Ind. Mayor Pete Buttigieg at 9 percent, Minnesota Sen. Amy Klobuchar at 5 percent, former New York City mayor Michael Bloomberg at 4 percent, and businessman Andrew Yang at 3 percent.

Both Hawaii Rep. Tulsi Gabbard and New Jersey Sen. Cory Booker have the support of 2 percent of Democratic primary voters, while the remainder of the candidates — businessman Tom Steyer, former Housing Secretary Julian Castro, Colorado Sen. Michael Bennet, former Maryland Rep. John Delaney, former Massachusetts Gov. Deval Patrick and author Marianne Williamson — receive 1 percent support or less.

The poll comes as seven Democratic candidates prepare for their sixth primary debate Thursday night. The seven candidates who have met the DNC threshold to participate are: Biden, Sanders, Warren, Buttigieg, Klobuchar, Steyer and Yang.

The poll’s margin of error among Democratic primary voters is +/- 4.84 percentage points.

Older voters, African Americans continue to be Biden’s key base

Biden’s strength in the poll is largely due to his support among African Americans (50 percent support him, compared with 15 percent for Warren and 8 percent for Sanders), and Democrats over 50 (38 percent back him, compared with 14 percent for Warren and 7 percent for Sanders).

Republican pollster Bill McInturff, whose firm conducted the poll along with Democratic firm Hart Research Associates, says those groups form the backbone of Biden’s durability as a candidate.

“Joe Biden is sitting on a stable result, with a stable political base that has shown no inclination to vote for two of the other candidates [Sanders and Warren],” McInturff said, adding that both Sanders and Warren are also well-positioned to remain competitive as the race goes on.

As the Democratic primary race competes with wall-to-wall coverage of the impeachment of President Donald Trump, Biden is also a favorite among Democrats who prefer the party to focus on impeachment now rather than looking to the ballot box in 2020.

Of the 45 percent of Democratic voters who say the party should focus on removing Trump from office through impeachment, Biden leads by double digits. Among the 46 percent who say the party should pay more attention to efforts to oust Trump in November 2020, however, the three top candidates are essentially tied.

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Biden also performs particularly well with Democratic voters who say they prefer smaller-scale policy changes that might be less costly and easier to pass than larger-scale proposals. Of the four-in-ten Democratic voters who prefer smaller-scale change, Biden gets 39 percent support. (Only one other candidate — Pete Buttigieg — breaks double digits with those voters).

But of the 56 percent of Democratic voters who say they prefer larger-scale policies that may be harder to pass but would create major change, Sanders receives 32 percent support and Warren gets 25 percent. That’s compared with just 18 percent for Biden.

Sanders has enthusiasm edge

While the former vice president’s lead has been durable, it’s Sanders who now enjoys the most enthusiastic base.

A third of Democratic primary voters — 33 percent — say they’re enthusiastic about Sanders’ candidacy, compared with 29 percent for Warren and 26 percent for Biden.

And among those who say Sanders is their first choice for the nomination, 58 percent say they’ll definitely vote for him, while the remainder say they’ll probably vote for him (27 percent) or are merely leaning toward supporting him at this time (15 percent).

That’s compared with 45 percent of Biden first-choice supporters who say they’ll definitely vote for him, 25 percent who say they probably will, and 28 percent who say they're just leaning toward it.

For her part, despite her recent dip, Warren still enjoys the lowest level of discomfort among Democratic primary voters.

A combined 26 percent say they either have some reservations about Warren or are uncomfortable with her candidacy, compared with 32 percent who say the same of Sanders and 35 percent who say the same of Biden.

And Warren is also the most popular second-choice candidate in the field, with 22 percent of primary voters picking her as their top alternative, compared with 17 percent for Sanders, 13 percent for Buttigieg and 12 percent for Biden.

Those advantages make for a race that could see some volatility once voters actually start making their preferences known at the polls.

“It’s easy to see the potential weakness for any of these three candidates. It’s easy to see their strengths. But it’s not easy to see how that really plays out over four months of actual voting,” McInturff said.

For Buttigieg, who has recently taken heat from progressive foes for his moderate policy positions and his previous work for a major consulting company, about half of Democrats are either enthusiastic (18 percent) or comfortable (32 percent) with him, while 31 percent say they have reservations or are uncomfortable.

That 31 percent is a slight uptick from September, when 27 percent of primary voters expressed concerns about the South Bend mayor.

Bloomberg, who entered the race last month, debuts in this survey at 4 percent support, besting many of the race’s more established candidates after spending more than $100 million of his personal fortune on television ads.

But the bad news likely outweighs the good for the billionaire newcomer; more than half of Democratic primary voters (52 percent) say they are very uncomfortable with his candidacy or have reservations about it, and not a single supporter of either Sanders or Warren picked Bloomberg as a potential second choice.

Single-payer Medicare for All still popular with Democrats

As the Democratic candidates continue to draw battle lines over health care policy, the survey finds that a single-payer government health care plan that would eliminate private insurance garners the support of nearly seven-in-ten Democratic primary voters — 68 percent. In September, 63 percent said the same.

But the appeal of such a program is limited outside the Democratic base; just 44 percent of all adults say they back that proposal, including 48 percent of independents and just 14 percent of Republicans.

Unlike the single-payer plan, a proposal to allow all adults to buy in to Medicare — just like buying private insurance — is overwhelmingly popular both inside and outside the Democratic base, with 72 percent of all adults and 81 percent of Democratic primary voters saying they support such a policy.

The NBC/WSJ poll was conducted Dec. 14-17 of 900 adults — more than half of whom were reached by cell phone — and it has an overall margin of error of +/- 3.3 percentage points.

The margin of error for 410 Democratic primary voters is +/- 4.84 percentage points.