Democratic 2020 U.S. presidential candidates U.S. Senator Bernie Sanders (L) and U.S. Senator Elizabeth Warren during the first night of the second 2020 Democratic U.S. presidential debate in Detroit Reuters

Sen. Bernie Sanders' 2020 campaign is on the upswing, and it's put a huge target on his back.

He's been at the top of recent polls in important early voting states, while also dominating in fundraising and nabbing endorsements from prominent progressive groups.

Though they've been cordial for most of the campaign season, Sanders and his fellow progressive Sen. Elizabeth Warren have taken the gloves off.

Meanwhile, Trump has turned his attention from former Vice President Joe Biden to Sanders, attacking him in emails and tweets.

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Sen. Bernie Sanders' 2020 campaign saw a major resurgence in the months following a heart attack in October that raised questions about whether he'd continue running.

He's been at the top of recent polls in important early voting states, while dominating in fundraising and nabbing endorsements from prominent progressive groups. Sanders is also trailing closely behind Vice President Joe Biden for the top spot in national polls.

The Vermont senator's campaign entered the new year as strong as ever, and with a major target on his back as a consequence. Sanders is being attacked from all sides.

Trump's focus has shifted from Biden to Bernie

Though President Donald Trump was impeached last month over a scheme to pressure Ukraine to dig up dirt on Biden, he's recently turned his attention toward Sanders - signifying he views him as an increasing and significant threat.

The Trump campaign has been bombarding supporters with emails warning them about the purported dangers of a President Sanders. In one such email earlier this month, the Trump campaign referred to Sanders as the "Democrats' 2020 front-runner," even though the self-declared democratic socialist has consistently been in second place in national polls.

Trump has also been going after Sanders on Twitter in the early days of 2020.

On January 12, the president tweeted: "Wow! Crazy Bernie Sanders is surging in the polls, looking very good against his opponents in the Do Nothing Party. So what does this all mean? Stay tuned!"

In a tweet replying to Trump, Sanders said: "It means you're going to lose."

Meanwhile, after months of playing nice, Sen. Elizabeth Warren of Massachusetts and Sanders have apparently decided that the time has come for them to duke it out for voters on the Democratic left.

In recent days, Warren alleged that during a private meeting with Sanders in 2018, he told her that a woman could not win the election. Sanders has vehemently denied the charge, including when questioned about it during the first Democratic presidential debate in 2020 on Tuesday.

Moments after Sanders' denial during the debate, Warren doubled-down on the allegation while standing a few feet away from him.

CNN went after Sanders during the first Democratic presidential debate in 2020

After Tuesday's debate, in which CNN framed questions in a manner that was critical of Sanders, a number of outlets suggested the network was biased against the Vermont Senator.

CNN, for example, presented Warren's assertion that Sanders said a woman couldn't win as a fact just moments after he'd once again denied the allegation.

At the end of the debate, the two had a brief exchange that ended without a handshake, which sparked a great deal of speculation on cable news and social media.

Sanders on Wednesday told a reporter he discussed "the weather" with Warren during that viral moment.

Subsequently, CNN released a recording of the exchange that showed Warren accusing Sanders of calling her a liar.

"I think you called me a liar on national TV," Warren could be heard saying in the clip.

"What?" Sanders responded.

"I think you called me a liar on national TV," Warren repeated.

"You know, let's not do it right now. If you want to have that discussion, we'll have that discussion," Sanders then said. Warren replied, "Anytime."

It's clear that the gloves are off.

Despite the slew of attacks, Sanders' campaign on Tuesday night said it had its "best single day for fundraising on a debate day" so far. The campaign raked in a total of $1.7 million by the end of the night, from more than 100,000 contributions.

With his campaign on the upswing, it seems likely that the recent trend of attacks on Sanders will also increase.

But this could also benefit Biden, who could coast along to the nomination as Sanders and Warren fight over voters on the Democratic left.

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