Sen. Bernie Sanders Bernie SandersThe Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power Bernie Sanders: 'This is an election between Donald Trump and democracy' The Hill's 12:30 Report: Trump stokes fears over November election outcome MORE (I-Vt.) took a shot at Sen. Elizabeth Warren Elizabeth WarrenHillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns 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 Democratic senators ask inspector general to investigate IRS use of location tracking service MORE (D-Mass.) on Wednesday, a sign that the fight to be the Democratic Party’s progressive standard-bearer is heating up as Warren rises in the polls.

Sanders, referencing a Politico article about how centrist Democrats are flocking to Warren’s campaign in an effort to ensure he doesn’t win the nomination, tweeted that the “corporate wing” of the Democratic Party is intent on electing anyone but him.

“The cat is out of the bag,” Sanders tweeted. “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.”

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. https://t.co/zimci7JRO6 — Bernie Sanders (@BernieSanders) June 19, 2019

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The eruption from Sanders comes as Warren has caught or surpassed him in several national surveys and polls of early-voting states, such as Nevada and South Carolina.

While only a light jab, Sanders’s tweet is likely a preview of the all-out brawl brewing on the left as he and Warren battle for the progressive mantle in a crowded Democratic primary of 24 candidates.

Sanders on Wednesday night denied the tweet was aimed at Warren, saying it was intended to be about the think-tank Third Way.

“That tweet was not about Elizabeth Warren at all. Not at all. Elizabeth is a friend of mine, and we’re going to run what I hope are issue-oriented campaigns," Sanders told CNN's Chris Cuomo on "Cuomo Primetime."

The campaigns for Sanders and Warren have largely played nice or ignored one another so far.

In an interview with The Hill on Tuesday — before the tweet dinging Warren — Sanders’s campaign manager Faiz Shakir said he wasn’t worried about the Massachusetts senator’s polling gains, arguing progressive voters would come home to Sanders once the field shrinks.

“To be distracted by the latest ups and downs of poll numbers would be a disservice,” Shakir said.

At the time, Shakir and other Sanders aides refused to swipe at Warren, instead crediting her success to “the hard work of her and her staff.”

“I’m not going to criticize that,” Shakir said.

Asked if Sanders and Warren were standing in each other’s way, Shakir said, “I don’t even know the answer to that.”

He would only say that Sanders would look to draw a distinction between himself and the other candidates, including Warren, “once we crystalize our visions.”

Several prominent Sanders surrogates have been taking subtle jabs at Warren over Twitter, questioning her polling and casting her message as democratic socialism-lite.

But the Sanders tweet is a clear sign to his fervent base of supporters that it’s open season on Warren ahead of the first Democratic debates next week in Miami, although the two will appear on stage on different nights.

His tweet was sent in close proximity to a memo from Shakir, who said that the “corporate wing” of the Democratic Party had “declared war” on Sanders.

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Shakir pointed to the same Politico article, which said centrist Democrats are in part responsible for Warren’s surge in the polls. And he noted that Third Way, a centrist Washington think tank, said recently it would support any Democratic candidate except for Sanders.

“Leading the fight against Senator Sanders is Third Way, a Washington think tank that has admitted that its major source of funding is the same reckless financial industry whose greed and illegal behavior created the Great Recession which devastated millions of American lives, and that Senator Sanders has been challenging for his entire public life,” Shakir said. “As our campaign fights to unify the Democratic Party in support of basic economic rights and to defeat Donald Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE, our response to our corporate opponents’ animosity is exactly what FDR stated in 1936: ‘We welcome your hatred.’ ”

Some Democrats, such as Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE’s former spokesman Brian Fallon, criticized Sanders, saying he’d allowed the remarks from Third Way to become a divisive wedge issue in the primary.

“I thought everybody just agreed they weren’t going to let Third Way’s attempt to start a fight between Sanders and Warren work,” Fallon tweeted. “Sanders is taking the bait.”

One Democratic strategist who is not affiliated with any campaigns said Sanders’s comment shows he’s concerned by Warren’s polling gains.

“They know she’s gaining, and in some places already passing him, and they don’t have a good plan to counter it,” the strategist said. “Yelling about Third Way or the establishment has gotten him very far in the past, but Warren is passing him not just because she has a lot of new policy ideas, but because she is running a much better campaign while they’re stuck playing the hits from last time.”

A Warren aide said the Massachusetts senator would double down on her current strategy, with a focus on holding town halls and press events to continue spreading her message about her plans.

“It’s clearly working,” a Warren ally said. “I don’t think anyone has gained in the way she has.”

A Monmouth University survey released Wednesday found former Vice President Joe Biden Joe BidenPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Fox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio MORE with a solid lead over the rest of the field, at 32 percent support.

But Warren was by far the biggest gainer in the poll, jumping 5 points to 15 percent support and passing Sanders, who is down 1 point to 14 percent support. Warren’s bump came largely from an 11-point increase in support from self-identified liberals.

As Warren has risen in the polls, the Sanders campaign has vigorously fought back against the growing narrative that the liberal energy in the party has shifted from him to her.

The Sanders campaign has been making an electability argument for Sanders, pointing to polls that show he and Biden routinely post the largest margins over Trump in head-to-head matchups. Warren’s head-to-head numbers with Trump have not been as strong.

Still, even some Sanders supporters say the attack on Warren has a whiff of panic about it.

Jonathan Tasini, a progressive strategist who strongly supports Sanders and will vote for him, called it “dumb and unwise” to cast Warren in anything other than a positive light.

“Bernie needs to expand his base and it makes no sense to react to a poll in June 2019 to tarnish Warren and alienate a significant slice of voters she is appealing to, especially women, because six months from now, you might need those voters who will flip you the bird if they sense you had a hand in wounding her, not to mention needing them to be enthusiastic to vote in the general election if you are the nominee,” Tasini said.

Updated at 8:04 p.m.