Sens. 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 Bernie Sanders Bernie SandersSirota reacts to report of harassment, doxing by Harris supporters Republicans not immune to the malady that hobbled Democrats The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election MORE (I-Vt.) are battling to be seen as the main alternative to former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE in the Democratic presidential race.

Warren has been climbing in polls in recent weeks, surpassing Sanders in some surveys.

But Sanders, who in 2016 was the progressive alternative to front-runner Hillary Clinton Hillary Diane Rodham ClintonBloomberg 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 Trump pledges to make Juneteenth a federal holiday, designate KKK a terrorist group in pitch to Black voters MORE in the Democratic presidential race, is holding relatively firmly in the polls. He’s been ahead of Warren and in second place in some surveys, and his team continues to believe it is being underestimated.

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At the next presidential debate on Sept. 12, Warren and Sanders will bookend Biden, who has retained the lead in national polls. It will be an opportunity for them to go after the front-runner or take on each other, even as sources on both sides say they will once again form a united front.

Warren and Sanders — who speak privately from time to time and had one longer conversation at the end of last year, sources say — have avoided attacks on each other for the most part so far, but political observers question how long the détente will last.

“The race as I see it at this point is all about who will be Biden’s main opponent and, more importantly, who will win the hard-fought match-up between Elizabeth Warren and Bernie Sanders,” said one Democratic strategist who is unaffiliated with any campaign. “It’s shaping up to be Biden versus a progressive, and that appears to be Elizabeth Warren at the moment. But does Bernie come back? And who becomes the alternative? That's the biggest question.”

Democratic strategist Chris Kofinis also said the Warren-Sanders battle is intriguing, particularly in the post-Labor Day stretch as the race intensifies and inches closer to the early states' contests.

“There is supposedly a nonaggression pact between Warren and Sanders, but those pacts have a way of collapsing under pressure,” he said.

Both Warren and Sanders have their strengths.

Sanders has built a solid infrastructure over the past four years, with email lists that include a powerful small-donor base that helped him win nearly $6 million on the day he launched his campaign.

His influence has helped shift the Democratic Party to the left and can even be seen to some degree in the power of rising progressive stars such as Rep. Alexandria Ocasio-Cortez Alexandria Ocasio-CortezOn 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 Anxious Democrats amp up pressure for vote on COVID-19 aid MORE (D-N.Y.). Debates about "Medicare for All," the Green New Deal climate plan and free college tuition were all spearheaded by Sanders and have become mini-mantras for progressives.

Team Sanders says its candidate is performing better than Warren in Rust Belt states, and it argues he would be a stronger challenger to President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE in the general election.

“Our movement is built of people with lived experiences who understand the injustices in our country,” Sanders campaign manager Faiz Shakir said in an email to The Hill this week. “It is a mission to transform America that guides this campaign and will ultimately bring us to victory.”

One Sanders ally acknowledged that there is a competitive element between the two camps on who can be the alternative to Biden.

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Still, the ally acknowledged that “most people aren’t going to jump off a cliff if it’s one or the other.”

“Of course I’d prefer Bernie, but I would be jumping up and down if she won too,” the ally said.

Warren’s campaign also has huge strengths.

It is seen as the side with the most momentum, and big crowds that have gathered for Warren events in St. Paul, Minn., and Seattle have lent the sense that she is becoming this year’s inspirational Democratic candidate.

Warren aides and allies say the trend lines are working in their favor. The point to the overall trajectory of the campaign: her string of proposals, her growing crowd sizes and polls that show she is making significant gains with both young and older voters — not just younger men who support Sanders.

“You know her rationale for running,” one Warren aide said. “It has started to really break through.”

Democratic strategist Basil Smikle, who served as the executive director of the New York State Democratic Party and is a former aide to Clinton, says the fight is very much on.

He said it’s clear that while Biden has a following, many Democratic voters would prefer another candidate.

“Polls suggest Biden has about one-third of the primary electorate, which means two-thirds want someone else,” Smikle said.

“Warren and Sanders are battling to be an alternative to Biden with a coalition of young progressives,” he added. “Both still need to wrest some older voters, particularly African Americans, from Biden’s stronghold on these constituencies.”

Biden has long been seen as a weak front-runner vulnerable to a strong challenge.

“I don’t think people are sold on him,” the Democratic strategist said. “So they’re immediately going to look for that other person.”

Over the course of the last month, Biden has made headlines for a string of gaffes, including this week when The Washington Post reported that he repeatedly confused facts in a story about the awarding of a medal to a military hero.

“I think his numbers will go down the closer we get to Iowa,” predicted Adam Parkhomenko, a Democratic strategist who served on Clinton's 2008 and 2016 campaigns. “Warren’s support has been building and picking up along the campaign. Her support is far more concrete than Biden’s.”

Other strategists see it differently.

They suggest either Sanders or Warren will fall by the wayside and that the other will be the rival to Biden.

"It'll be just like 2008 and 2016," one strategist said. "It will be the front-runner, Biden, and someone else. The question now is, who?"