(CNN) Sen. Bernie Sanders has company in the rapidly escalating race-within-a-race to lead the progressive wing in the 2020 Democratic presidential primary: Sen. Elizabeth Warren .

A series of recent polls have shown the Massachusetts senator on the rise, placing slightly behind Sanders in Iowa and Florida , and leading him in a recent survey out of Nevada . A pair of nationwide polls released Wednesday showed them either in a statistical dead heat or with Sanders' holding a small advantage

Warren's uptick has complicated Sanders' efforts to cast himself as the Democratic left's foil to former Vice President Joe Biden, the unapologetic moderate and early polling leader in a sprawling, 23-candidate field.

And her rise also muddies Sanders' path to the nomination. For most voters, there isn't much clear space between them on the major policy questions dominating debate within the party. And even among progressive activists, the places where Sanders and Warren split can appear trifling in contrast to Biden's "return to normalcy" message.

Increasing support for Warren has also provided a new opportunity for the anti-Sanders crowd, worried over his calls for "political revolution," to criticize him, while openly discussing the Massachusetts senator in a different light. Jim Kessler, a co-founder of the centrist think thank Third Way, tweeted favorably about her recent plan to aid minority entrepreneurs.

In an interview with CNN, Kessler praised Warren's campaign as "policy rich" and criticized Sanders' speech defending democratic socialism.

Warren's proposals are "within the lines of Democratic policies. They're not democratic socialist policies. They're within the lines of a candidate who says she's a capitalist," Kessler said. "Our view is that it passes that test overall."

Third Way's friendly words, which also featured in a Politico report early Wednesday, caught the attention of the Sanders campaign. A few hours after the story began to circulate, campaign manager Faiz Shakir tweeted out a familiar criticism of the group.

"Third Way's approach antagonizes no one, stands up to nobody and changes nothing," Shakir wrote, echoing a comment Sanders made 10 days ago in Iowa about his moderate rivals. "This is a Washington think tank that takes Wall Street money, so if @ThirdWayTweet is the opposition to @BernieSanders' campaign, which is leading Trump in poll after poll, we welcome the contrast."

About an hour later, Sanders himself chimed in, tweeting a link to the initial report and commenting: "The cat is out of the bag. The corporate wing of the Democratic Party is publicly 'anybody but Bernie.' They know our progressive agenda of Medicare for All, breaking up big banks, taking on drug companies and raising wages is the real threat to the billionaire class."

On Wednesday night, Sanders told CNN's Chris Cuomo, "That tweet was not about Elizabeth Warren," saying it was directed only at Third Way.

The centrist group's stamp of modest approval for Warren offers new credibility to a narrative that has been tossed around among progressives since the early days of her campaign -- the idea that, by effectively running slightly to Sanders' right, she could end up providing a more comfortable option for Democrats who, even as they want to push left, might be uneasy with some of Sanders' more radical views.

But there is also peril here for Warren. Progressives have a long-running feud with Third Way and similar centrist groups. Kessler's complimentary words offer Sanders an opportunity to amplify differences between the two candidates without lodging the kind of direct, and potentially self-defeating, attacks that he has so far avoided.

It's no secret that Democrats widely fear a slash-and-burn primary that hobbles the eventual nominee ahead of a general election showdown with President Donald Trump. But some progressives carry parallel concerns -- specifically, that a hard rift between Warren and Sanders could ultimately undermine their respective bids to run down Biden, while doing untold damage to a movement both have done so much to shape and advance.

Warren's campaign on Wednesday played down the polls and talked up her campaign infrastructure.

"We don't pay much attention to the polls," a senior campaign aide told CNN. "They will go up and down throughout the race and focusing on the daily headline, tweet, or cable news chatter is not a recipe for long term success. Every day we are building an organization, talking about problems facing the country, and Elizabeth's solutions for big structural change."

Sanders and his top aides have repeatedly sidestepped direct questions about Warren, noting accurately that most progressive voters don't view the primary as a binary choice between the two, while trying to keep the focus on a larger picture -- one that shows Sanders handily beating Trump, by about the same percentages as Biden, in a head-to-head matchup. Warren also leads Trump in similar matchups, but by narrower margins.

In an email memo to supporters on Monday, Shakir touted "our strong grassroots campaign" while listing and analyzing a "spate of polls" he said underscored the strength of Sanders' base and his "solidified" place "as the candidate in the strongest position to defeat Donald Trump in the general election."

Among the surveys cited was a Fox News poll from this weekend that showed both Sanders and Biden comfortably defeating Trump: Biden by 10 points, 49% to 39%, and Sanders by 9, 49% to 40%. Warren, California Sen. Kamala Harris and South Bend, Indiana, Mayor Pete Buttigieg also led in their matchups, but by narrower margins. Warren held a two-point advantage, 43% to 41%. Harris (42% to 41%) and Buttigieg (41% to 40%) led by a single point.

"The challenge here is to beat Trump and to nominate and then elect someone in the Democratic primary process who's going to move the country in a progressive direction -- period," Sanders campaign senior adviser Jeff Weaver told CNN last week. "So the question is, Who is best able to do that? And I think that as long as the debate is a principled debate on policy positions, on one's ability to appeal to a wide range of voters and not personal and nasty ... I think that is a healthy debate to have in the Democratic Party."

Though Sanders and his most visible advisers have shown little appetite, more than seven months out from the Iowa caucuses, for any public jousting with Warren, some of his other aides -- most often active on social media -- have been more willing to highlight the candidates' differences.

On Sunday, Sanders campaign co-chair Nina Turner, retweeted and quoted in part from Washington Post opinion columnist Elizabeth Bruenig's recent piece addressing those distinctions.

"There's little for a progressive stalwart to object to in Warren," Bruenig wrote, after ticking off some of the Massachusetts senator's proposals and positions. "But there's still a distinction to be drawn between her approach and Sanders's, and much of it comes down to the matter of regulation vs. revolution."

"Indeed!" Turner tweeted . "This is about transcendence for the people!"

Others, like Sanders policy adviser Warren Gunnels and campaign speechwriter David Sirota, have pushed back -- again, on Twitter -- against the suggestion that Warren stands alone in the Democratic primary's policy engine room.

"For the record, @BernieSanders has 25 policy proposals on his website," Gunnels wrote on Sunday as he began a thread that included links to an assortment of plans and legislation calling for everything from Sanders staples like Medicare for all and a $15 minimum wage to the "Loan Shark Prevention Plan" he introduced in May with New York Democratic Rep. Alexandria Ocasio-Cortez.

In a recent interview, Shakir -- who, like some Warren staffers, is a veteran of former Democratic Senate Majority Leader Harry Reid's office -- would only say that Warren's campaign "deserve(s) credit for the success they're getting and the hard work that she's putting in their campaign. That's great. They should continue to do whatever they think will help them win."

For now, both candidates seem comfortable pursuing their own paths -- with Warren strategically rolling out her policy proposals and Sanders personally confronting corporate powers, as he did at the recent Walmart shareholders meeting . The question up ahead, as Justice Democrats communications director Waleed Shahid put it, is what they plan for their next acts.

"They've really centered the policy and ideology, now they are going to have to make the case why they're the candidate to lead the American people," Shahid said. "I'm curious where they go."