Sen. Bernie Sanders Bernie SandersTrump, Biden will not shake hands at first debate due to COVID-19 Sanders tells Maher 'there will be a number of plans' to remove Trump if he loses Sirota reacts to report of harassment, doxing by Harris supporters MORE (I-Vt.) is rising in the polls ahead of Thursday’s pivotal debate in Los Angeles, reestablishing his standing in the top tier of Democratic contenders with the Iowa caucuses less than 50 days away.

Sanders, who electrified liberals over the course of his unlikely challenge to former Secretary of State Hillary Clinton Hillary Diane Rodham ClintonThe Memo: Trump furor stokes fears of unrest Bloomberg rolls out M ad buy to boost Biden in Florida Hillicon Valley: Productivity, fatigue, cybersecurity emerge as top concerns amid pandemic | Facebook critics launch alternative oversight board | Google to temporarily bar election ads after polls close MORE in the 2016 Democratic primary, has at times been treated as an afterthought in the 2020 race, which has produced a rival on the left in Sen. Elizabeth Warren Elizabeth WarrenOvernight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds On The Money: Half of states deplete funds for Trump's 0 unemployment expansion | EU appealing ruling in Apple tax case | House Democrats include more aid for airlines in coronavirus package Warren, Khanna request IG investigation into Pentagon's use of coronavirus funds MORE (D-Mass.) and a new Democratic star in South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBillionaire who donated to Trump in 2016 donates to Biden The Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November Buttigieg stands in as Pence for Harris's debate practice MORE.

But party leaders and the news media are taking notice now that Warren has slipped in the polls. Buttigieg is sustaining attacks from every direction and questions linger about whether former Vice President Joe Biden Joe BidenBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Joe Biden should enact critical government reforms if he wins MORE can go wire-to-wire as the front-runner.

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Sanders appears to be hitting his stride at just the right moment, surging past Warren and cutting into Biden’s lead in new national surveys. Sanders leads in the RealClearPolitics average of polls in New Hampshire, and is in second place in Iowa, only 3 points behind Buttigieg.

Campaign officials say Sanders weathered the rough stretches on the strength of his firm base of support and unparalleled grassroots fundraising operation.

The campaign did not panic or make dramatic changes in messaging or strategy in the fall after Sanders suffered a heart attack and appeared to be headed for a disappointing finish.

Instead, the campaign banked on Sanders’s unwavering focus on economic issues and wealth inequality, believing that consistent message would win out in the end.

The campaign believes the strategy is paying off in the stretch run to Iowa, leading to rising poll numbers, record fundraising and big crowds.

“No other candidate has as durable a base as we do,” said Nina Turner, the former Ohio state senator who has been one of Sanders’s most high-profile surrogates since 2015.

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“So now he has an energized base and we’re starting to see his crossover appeal. We can enumerate that too, with 4 million donations and hundreds of thousands of volunteers. We have the receipts and we have the moral clarity from a senator who has stood on the right side of justice for over 40 years, whether it’s been popular or not.”

Nationally, Sanders is back in the game, surpassing Warren after trailing her by double-digits, and cutting into Biden’s lead at the top.

The latest NPR-PBS-Marist national survey released Sunday found Biden with the support of 24 percent of respondents, Sanders at 22 percent and Warren at 17 percent, with the Vermont senator leading among progressives, independents, men, nonwhite voters and young people. Sanders has a 20-point lead over the next closest contender among voters under the age of 45.

Sanders has also raised more money than anyone else while reaching the 4 million donors milestone in record time.

And polls routinely show that Sanders’s backers are the most enthusiastic and most likely to have firmly made up their minds. Supporters are flocking to his town hall events. He has had the largest one-day crowds in both Iowa and New Hampshire, where high-profile surrogates helped him to attract record numbers.

“We knew he had a core base that looked like it might be in the 10 percent range, but he’s honing on 20 percent now and we know from experience in the Iowa caucuses that you can turn that into much bigger numbers when ballots are cast,” said Patrick Murray, the pollster for Monmouth University. “He’s not surging statistically, but his numbers are very, very solid, and that’s a big advantage when the other candidates are moving up and down.”

In Iowa, the Sanders campaign believes its enthusiastic base of support is ready to deliver a surprise victory on Feb. 3.

The campaign’s network of volunteers, led by young people and college students, knocked on 30,000 doors in 48 hours over the weekend.

A Sanders event in rural Ottumwa, Iowa, drew 200 people on Sunday night during a snowstorm.

“We’ve had a consistently strong field organization and presence here, but there’s no doubt that we’re starting to see the kind of momentum you get when good news builds on good news,” said Bill Neidhardt, the campaign’s deputy director in Iowa.

Sanders also has a narrow lead in two of the past three polls of New Hampshire, where he posted a blowout victory against Clinton in 2016.

On Friday, Sanders and Rep. Ilhan Omar Ilhan OmarOmar urges Democrats to focus on nonvoters over 'disaffected Trump voters' Omar fires back at Trump over rally remarks: 'This is my country' Trump attacks Omar for criticizing US: 'How did you do where you came from?' MORE (D-Minn.) turned out 1,300 people at a rally in Nashua, which is the largest crowd for any candidate in New Hampshire this year.

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And with the next debate in liberal California, the Sanders campaign is turning its sights on the Super Tuesday state with the largest delegate haul in the nation.

Sanders will attend five events across California this week, including a Los Angles rally with Rep. Alexandria Ocasio-Cortez Alexandria Ocasio-CortezWill Democrats attempt to pack the Supreme Court again? On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline McCarthy says there will be a peaceful transition if Biden wins MORE (D-N.Y.). The campaign has five offices in the state and says it has surpassed 8 million attempted voter contacts through its network of 760,000 volunteers.

A Los Angeles Times survey from early December found Sanders passing Warren to take a narrow lead in the Golden State.

There are still questions about whether Sanders can win the nomination.

While Sanders has built a more diverse coalition than he had in 2016, Biden continues to have a huge lead with black voters, who are the cornerstone of the party’s primary electorate.

And while polls find Sanders running as good or better than anyone in head-to-head match-ups against President Trump Donald John TrumpBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Pelosi: Trump Supreme Court pick 'threatens' Affordable Care Act MORE, many Democrats remain fearful that nominating a self-described socialist will be a surefire general election loser.

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If Sanders emerges as the rival to Biden as the campaign enters the home stretch, there are questions on whether he can emerge on top.

Former President Obama has reportedly told those close to him that he’d speak out against Sanders if he continues to build momentum, and some believe that other party leaders would join a concerted “anyone but Bernie” effort to block him from winning the nomination, if it comes to that.

Many mainstream Democrats continue to be annoyed by Sanders’s persistent lines of attack against the national party and the moderates who refuse to embrace the left’s most ambitious policy proposals.

“He’s still running the same divisive campaign and claiming that it’s all rigged against him,” said Jon Reinish, a Democratic strategist. “So it’s no surprise that you’d have a popular party figure like President Obama speak out, and he’s smart to raise the alarm about what’s at stake here.”

The Sanders campaign says it’s fueled by the doubters and by slights from the political press. Sanders allies have made a pastime out of highlighting examples of media outlets ignoring or downplaying Sanders’s standing in the polls.

“It’s no surprise; the establishment will do what it’s always done to protect the status quo,” said Turner. “We’re ready for it. Thank God our victory won’t be contingent on the establishment or media elites.”