Sen. Bernie Sanders' 2020 campaign is virtually stagnant, according to INSIDER polling, as some of his top rivals have rapidly gained territory and surpassed his levels of support with likely Democratic primary voters.

In mid-February, roughly 50% of likely Democratic primary voters familiar with Sanders said they'd be satisfied to have Sanders as the nominee.

Though that number rose to 57% for Sanders by mid-April, but dropped all the way down to 49% by early June.

Meanwhile, Sen. Elizabeth Warren, a top progressive rival of Sanders', saw her numbers go from 39% in mid-February to 55% in early June.

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Sen. Bernie Sanders started off his 2020 campaign with a bang, dominating the competition in polling and fundraising, but now he's treading water as his top rivals rapidly gain territory.

INSIDER's recurring polling shows that Sanders' campaign hasn't made significant progress since he launched his campaign in February, and has more or less hit a wall when it comes to levels of support. This is made worse by the fact that Sanders has nearly universal name recognition, meaning that the senator from Vermont has few new audiences to persuade.

INSIDER has asked a recurring series of poll questions through SurveyMonkey Audience since late 2018, specifically inquiring as to which contenders respondents would be satified with in the event they became nominee.

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In mid-February, roughly 50% of respondents aware of Sanders, who said they're likely to vote in a Democratic primary and said they're already registered to do so, said they'd be satisfied to have the Vermont senator as the presidential nominee — and by mid-April that number rose to 57%. But after former Vice President Joe Biden hopped in the race in late April, Sanders' numbers started to drop and went down to 49% by early June.

Biden is the undisputed frontrunner, and INSIDER data suggests it will be very hard for the other candidates to chip away at his lead.

Sanders is in a uniquely difficult position. Lesser-known rivals lack the cachet among voters that Sanders cultivated after already making a run at the White House, which means they have new audiences to persuade. Sanders is a known quantity, and despite having a connection with many voters through his ample mailing lists has not been able to translate those assets into any measurable improvement in support among those familiar with him.

Meanwhile, candidates like Sen. Elizabeth Warren of Massachusetts, who perhaps represents the biggest challenge to Sanders with progressive voters, have seen their numbers go up.

In mid-February, about 38.5% of likely Democratic primary voters aware of Warren said they would be satisfied with her as the Democratic nominee. That rose to roughly 55% by early June.

Warren has launched ambitious policy proposals on a host of subjects and garnered significant media attention for them. The poll shows she is turning people who know about her into fans or at least supporters — Sanders has not been able to do that.

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Sanders could get a boost from the first Democratic debate at the end of the month, but his campaign has already received a significant amount of media attention so it's hard to point to a lack of exposure as the cause for his flatlining numbers.

With that said, while his recent numbers spell trouble for Sanders' campaign, it's still quite early in the campaign season and the democratic socialist has plenty of time to rebound.