That approach was on display Monday as Sanders introduced a proposal to cancel all student debt across the country. His plan came less than two weeks after Warren announced her own idea for scrapping student debt, one that would cover fewer people than Sanders's would. "Under the proposal we introduced today, all student debt would be canceled in six months," Sanders said.
But doubling down on his ideological purity and socialist credentials carries risks for the senator from Vermont, other Democrats say. It's enabled Warren to position herself as impassioned but reasonable, while Sanders holds down the leftward flank of the Democratic Party and serves as the ideological outlier in the race.
"They both have a crusader mentality around correcting what is wrong," said Rep. Pramila Jayapal, D-Wash., who knows both and has not yet endorsed in the contest. "They understand they are speaking to a similar vision of the country, and obviously they are trying to distinguish themselves from each other."
For Sanders, the threat from Warren, D-Mass., is different from any he's faced during his long political career, including in his 2016 presidential run, since Warren in many ways embraces the same brand of fiery liberalism.
Among some Sanders allies, there is private frustration that Warren is getting credit for ideas that they believe originated with him. And that frustration is starting to show.
Last Friday, Warren released a plan to end private prisons. That same day, Sanders adviser Josh Orton tweeted two articles about Sanders's own work on prison reform, from several years ago. "Flashback Friday," he wrote.
On Wednesday, Warren announced her endorsement of Tiffany CabĂˇn in the race for district attorney in Queens. Less than an hour later, Sanders campaign manager Faiz Shakir tweeted: "Just for everyone's general awareness: Bernie spoke with Tiffany CabĂˇn and was impressed. We had already communicated our endorsement, but per their request, we waited to roll it out until it best suited the campaign strategy."
One recent episode put Sanders in the position of insisting he hadn't meant to attack Warren. On Wednesday, a tweet from Sanders's account declared: "The cat is out of the bag. The corporate wing of the Democratic Party is publicly 'anybody but Bernie.' " It linked to a Politico article about centrists coming around to Warren as an alternative to Sanders.
Pressed by CNN host Chris Cuomo, Sanders said the tweet "was not about Elizabeth Warren at all" but about Third Way, the centrist organization that was critical of him in the story.
But the tweet looked to some like a dismissal of Warren as a candidate favored by the party's "corporate wing." Some Sanders allies thought it sent the right message - criticizing Third Way - but delivered it clumsily by inserting Warren's name into the mix, according to people familiar with the situation, who like others spoke on the condition of anonymity to describe private discussions.
Warren allies, meanwhile, are actively trying to convert supporters of Sanders and other candidates. The Progressive Change Campaign Committee, a group backing Warren, launched an initiative Monday called "Switch to Warren," and has set up a website sharing stories of voters who initially backed others but are now in Warren's camp.
The challenge for both candidates is that, since they're appealing to many of the same voters, any critiques must be handled delicately to avoid alienating possible backers. Warren has gained steam in the campaign by touting a fast-growing portfolio of proposals to shift resources from the wealthy to working-class Americans, appealing to many of the same rebellious voters who have fueled Sanders's ascent. "I have a plan for that" has become her mantra.
And at a moment when many Democrats are demanding greater diversity and fresher faces, Warren may have an edge as a woman and a first-time presidential candidate.
Sanders and his aides are essentially advancing two main arguments at this point in the race. One is that his history of fighting for liberal priorities is unequaled by other candidates - including, by implication, Warren.
"He was the standard-bearer for many, many, many issues which are now accepted in the Democratic Party and that his opponents support," said Sanders senior adviser Jeff Weaver. "It is important to know who was sort of the trailblazer on many of these things."
The other is that he is best equipped to beat President Trump, particularly in the upper Midwestern states that helped tilt the 2016 election toward the GOP. Sanders's supporters argue that his gritty brand of economic populism could appeal to some Trump supporters. "His record stands alone," said Ari Rabin-Havt, chief of staff for the Sanders campaign.
Sanders's relatively gentle approach to Warren, contrasting with his usual combative tactics, reflects complicated personal and political dynamics. Sanders has long counted Warren as a friend and ideological ally, and they have often supported each other's initiatives in the Senate.
The presidential contest has scrambled that landscape. "Both of them share the same path, which is to consolidate the progressive lane and expand a little bit beyond that," said Adam Jentleson, a Democratic strategist with friends in both camps. "Each of them needs the other's supporters to win, but the other's supporters are less likely to go to them if they are mean to each other."
A recent Monmouth University poll showed Warren running about even with Sanders - a big shift from the March survey, when he enjoyed roughly three times as much support as she did. The poll did not demonstrate conclusively that her gains came at his expense. But it did show that Warren's biggest jump came among liberals just as support for Sanders dropped with that group.
Before Warren's rise, Sanders's 2020 campaign confronted foes that were familiar from his first presidential run: wealthy Democratic donors, a think tank with ties to Hillary Clinton, and a leading candidate preferred by many party veterans, in this case former vice president Joe Biden.
All that changed when Warren started to make strides. While her momentum took some in the party by surprise, Sanders and his inner circle have long regarded her as a potentially formidable opponent. She came up in their private conversations well before the 2020 race started, according to people familiar with the discussions.
Just as Sanders has avoided going after Warren, she has steered clear of directly attacking him. The candidates will appear on different nights in this week's Democratic debate, postponing the possibility of a showdown on national television.
But behind the scenes, they are battling each other in different ways. Both, for example, are cultivating relationships with liberal Democrats on Capitol Hill whose endorsements would be valuable.
Sanders unveiled his student debt plan Monday alongside Jayapal and Reps. Alexandria Ocasio-Cortez, D-N.Y., and Ilhan Omar, D-Minn., rising stars of the left. Warren collaborated on her student debt proposal with Rep. James Clyburn, D-S.C., a kingmaker in a key early-primary state.
Perhaps nothing exemplifies their divergent strategies like their party affiliations. Sanders is an independent who identifies as a democratic socialist, giving him an advantage with voters who are suspicious of both parties and skeptical of the political establishment.
Warren, a Democrat, has emphasized that she is a capitalist, while arguing for using the tools of governance to level the playing field. That could give her an edge with traditional Democrats who worry about socialism and fear alienating centrist voters in the general election.
Their rhetorical styles in some ways mirror those strategies. Where Sanders is thunderous and sweeping, Warren is pointed and analytical, associates say.
"I've been very impressed with how she can turn a complicated thought into a very simple, straightforward declarative sentence, which is a gift," said Sen. Richard Blumenthal, D-Conn. , a liberal who agrees with both of them on most issues. "And he has the same quality of rhetoric-making, but it strikes me as a different style."
While Sanders has been cautious with Warren, he is taking a very different approach to his other main rival, Biden. Sanders is among the most vocal critics of Biden's record, comparing it disparagingly with his own positions on issues like trade and foreign policy.
Weeks ago, Sanders aides discussed whether he should mention Biden by name in a high-profile speech designed to sharpen the contrast with the former vice president, according to people familiar with the situation. Sanders didn't name Biden, but the intent of his message - the need to avoid a "middle ground" - was clear.
Rep. Ro Khanna, D-Calif., a national co-chair for the Sanders campaign, said the primary contest will come down to Sanders, Warren and Biden.
"I think this race is going to have parallels to 1968," said Khanna, noting that Sens. Robert Kennedy, D-N.Y., and Eugene McCarthy, D-Minn., competed for liberal support that year, while vice ;resident Hubert Humphrey consolidated establishment backers.
Biden is like Humphrey, he said, adding, "I'll leave it for others to say who is Kennedy and who is McCarthy."
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