President Donald Trump's 2020 campaign has in recent weeks focused attacks on the Democratic presidential contender Bernie Sanders.

The president has also sought to attract prospective Sanders voters by claiming there is a Democratic establishment plot to halt Sanders' candidacy.

Trump campaign sources told Politico that this was all part of a plot to secretly boost Sanders' candidacy because Trump's team considered Sanders easier to beat than Joe Biden or Elizabeth Warren.

Some Republicans, however, have warned that such a strategy could backfire and that Sanders could harness some of the antiestablishment anger that propelled Trump to the presidency in 2016.

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President Donald Trump usually talks about the Democrats vying to take him on in the 2020 presidential race only to attack them or ridicule their candidacy.

But there is one candidate he's spoken of with perhaps unexpected sympathy: Sen. Bernie Sanders of Vermont.

In a refrain of one of the themes of his 2016 campaign, Trump is claiming the Democratic Party establishment is conspiring to halt Sanders' candidacy, with the impeachment trial part of the plot.

"They are rigging the election again against Bernie Sanders, just like last time, only even more obviously," the president tweeted January 17.

"They are bringing him out of so important Iowa in order that, as a Senator, he sit through the Impeachment Hoax Trial," Trump wrote, referring to Sanders' obligation as a US senator to forgo campaigning to attend Trump's impeachment trial in Washington, DC.

Of course many of the Trump campaign's messages are negative — focusing on Sander's support for sweeping government welfare and spending programs that have long been considered too radical even for mainstream Democrats to campaign on.

But underlying both types of messaging is a strategy at play, sources close to the Trump campaign told Politico.

By talking about Sanders, whether sympathetically or negatively, Trump is trying to boost Sanders' profile and chances of being chosen as the Democratic presidential nominee.

According to the report, the president considers Sanders relatively easy to beat in a general election, susceptible to attacks as a radical socialist opposed to American values.

The report echoes one in The New York Times from earlier this month that also indicated Trump was seeking to boost Sanders' candidacy.

President Donald Trump at his reelection kickoff rally. AP Photo/Evan Vucci

Critics of Sanders think Republicans would unleash a wave of attacks against Sanders if he won the candidacy, focusing on controversial statements in his early writings — such as an essay in which he linked sexual repression to breast cancer — and his past support for radical leftists abroad.

But some Republicans aren't so sure — and think Trump may be massively underrating the threat posed by Sanders.

They point to Sanders' popularity in the Rust Belt states that were key to Trump's victory in 2016.

Analyses of the 2016 election results have indicated as many as 12% of Sanders supporters may have voted for Trump instead of the Democratic nominee Hillary Clinton, which some believe was key to Clinton's narrow defeat.

"While Sanders may be a socialist, his populist brand of rabble rousing mirrors Trump, and could cut into the new Republican blue collar base," the Republican strategist James Barnett wrote in The Hill earlier this month.

Sen. Tim Kaine, Hillary Clinton's running mate in her failed 2016 campaign. Mark Wilson/Getty Images

One top congressional ally of Trump pointed to Sanders as perhaps the president's most dangerous challenger, saying he would be able to harness the antiestablishment anger that propelled Trump to victory in 2016.

"Bernie Sanders poses the greatest risk because we are still in an antiestablishment era for presidential elections," Rep. Mark Meadows of North Carolina told Politico.

Trump himself expressed anxieties about a Democratic challenge with Sanders on the ticket in a closed-door meeting in 2018 with top Republican donors secretly recorded by Lev Parnas, an associate of Trump's lawyer Rudy Giuliani.

"I think Bernie as vice president would have been tougher," Trump said, referring to Clinton's decision to pick Sen. Tim Kaine to be her running mate in 2016. "He was the only one I didn't want her to pick."

So as the race for the Democratic nomination heats up and Sanders continues to challenge former Vice President Joe Biden and Sen. Elizabeth Warren as frontrunner, Trump's shift of focus to Sanders will continue to be seen as a dangerous gamble by some top Republicans.