Moderate Democratic think tank Third Way is urging Iowa Democrats not to vote for Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE as the Vermont senator surges in polls ahead of the first-in-the-nation presidential caucus.

"Iowa Democrats: Please don't do what [President] Trump wants you to do," Third Way wrote in a memo released Tuesday, disparaging Sanders's chances at winning if he takes on Trump in November.

It said Sanders's record and "controversial background" makes him the “easiest target for Trump to beat."

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“Of course, Trump will lie about any Democratic nominee, but with Bernie, much of what he’d say would be easily backed up. When political charges are supported by evidence, they are very tough to combat,” wrote Third Way, which has long opposed the democratic socialist's White House bid.

Third Way called out several of Sanders's progressive proposals, including his signature “Medicare For All” health care plan, that the think tank claims would make the progressive a vulnerable nominee. Several of Sanders's opponents have backed the same or similar universal health care plans, including Sen. Elizabeth Warren Elizabeth WarrenGOP set to release controversial Biden report Biden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt MORE (D-Mass.), who is not named in the Third Way memo.

“Committed Democrats, like us, would vote for Sanders against Trump. But no Democrat remotely as far left as Sanders has ever won the presidency, and swing voters in crucial states would reject the radicalism of Sanders' background and ideas,” Third Way said.

Polls show Sanders leading the Democratic pack in Iowa, and multiple surveys of battleground states have found him beating or close to Trump.

In response to Third Way’s memo, the Sanders campaign highlighted its “growing support” from a diverse group of working-class Americans.

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"Let’s be clear: our growing support results from working-class Americans from diverse backgrounds demanding an agenda that transforms our country. All Democrats, even Wall Street-funded groups like Third Way, should be ecstatic to witness this movement attracting new supporters to strengthen the party and expand the electorate,” Sanders campaign manager Faiz Shakir said in a statement.

“To win seats up and down the ballot, we need to generate excitement and enthusiasm that drives a huge voter turnout among working people, not stifle it to protect special interests. Bernie Sanders has demonstrated over the course of this primary that this campaign is able to do that — and that’s why Donald Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE is nervous."

Sanders and his grassroots-funded campaign have been gaining momentum in Iowa in recent weeks.

A Des Moines Register-CNN poll, widely considered the most authoritative in Iowa, released earlier this month showed Sanders in the top spot with 20 percent support.

Sanders and former Vice President Joe Biden Joe BidenBiden leads Trump by 36 points nationally among Latinos: poll GOP set to release controversial Biden report Can Donald Trump maintain new momentum until this November? MORE are in a statistical dead heat at the top of the field in Iowa, based on a CBS News poll released Sunday.

And an Emerson College poll released Sunday found Sanders with a commanding 9-point lead over Biden in Iowa.