Sen. Bernie Sanders blasted the “corporate wing” of the Democratic Party in a not-so-veiled attack on his opponent Sen. Elizabeth Warren amid reports she’s earning support among moderate Democrats.

“The cat is out of the bag. The corporate wing of the Democratic Party is publicly ‘anybody but Bernie,’” Sanders tweeted, quoting a story that details how Warren is gaining traction among moderate Democrats.

“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,” Sanders added.

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The attack is one of the first progressive-on-progressive splits this election cycle, with both Warren and Sanders previously appearing on the same side and united against the party’s establishment and its favored choice – Vice President Joe Biden.

But according to Politico, centrist Democrats, led by their dislike of Sanders, are moving toward Warren as a credible candidate to take on President Trump without pitting the moderate and progressive wings of the party against each other.

Centrist think tank Third Way, which previously criticized Warren, has now praised her for her policy suggestions.

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“One is a Democratic capitalist narrative,” Matt Bennett, a co-founder of Third Way, told Politico. “The other is a socialist narrative.”

Sanders campaign manager Faiz Shakir criticized the group earlier this week, accusing it of being an opposition to Sanders’ campaign as it’s beholden to the interests of Wall Street.

“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,” Shakir wrote in a tweet.

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Warren’s campaign has surged in the last weeks, virtually tying with Sanders’ campaign for the second place in terms of electability, according to the latest Monmouth University Poll. Biden continues to dominate the field.

Biden currently has the support of 32 percent of Democratic voters, while Warren’s support has jumped five points to 15 percent from 10 percent in May. Sanders holds onto 14 percent support.