Sen. Bernie Sanders’ campaign has been on the upswing for the past few months, and he’s faced attacks on all sides as a result.

Though Sanders’ critics have suggested he’s not a true Democrat, given he serves Vermont as an independent in the Senate, this just doesn’t seem to matter to younger, progressive voters.

After the Iowa caucuses, there’s growing evidence Sanders is more electable than his Democratic critics have indicated.

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“You know, he’s not registered as a Democrat, to the best of my knowledge,” former Vice President Joe Biden said of Sen. Bernie Sanders in late January.

This point has often been raised by Democratic critics of Sanders, who is running as a Democrat in the 2020 presidential election but represents Vermont in the Senate as an independent. Sanders, who faced similar attacks in 2016, signed a loyalty pledge to the Democratic Party last year under Democratic National Committee rules.

“I am a member of the Democratic Party,” the pledge said. “I will run as a Democrat, accept the nomination of my Party, and will serve as a Democrat if elected.”

This has apparently not been enough for some of the party’s longtime figures, such as the veteran Democratic strategist James Carville, who went on a viral rant on MSNBC last week decrying the direction of the party. Carville tied this to Sanders, a self-declared democratic socialist who has often been depicted as too radical by establishment Democrats.

In a subsequent interview with Vox, Carville said: “Look, Bernie Sanders isn’t a Democrat. He’s never been a Democrat. He’s an ideologue. And I’ve been clear about this: If Bernie is the nominee, I’ll vote for him. No question. I’ll take an ideological fanatic over a career criminal any day. But he’s not a Democrat.”

But at a time when the Democratic Party is becoming increasingly progressive and moving more and more to the left, party loyalty may matter only to a certain cohort of older, more moderate or conservative voters.

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Left-wing Democrats don’t care about party loyalty

For the past year, Insider has been conducting a series of SurveyMonkey Audience polls to gauge Americans’ opinions on 2020 Democratic primary. You can download every poll here, down to the individual respondent data. (Read more about how the Insider Democratic primary tracker works here).

Seventy percent of Democratic voters who identify as “very liberal” like Sanders, according to the past eight polls conducted, ranging from late November to early February.

Comparatively, 41% of “very liberal” Democrats like Biden, who with decades of experience in Washington is perhaps more associated with the Democratic Party than any other 2020 Democrat.

Sanders is also slightly more popular with Democratic voters who identify as moderately liberal compared with Biden, with nearly 56% of this group saying they like the Vermont senator compared with about 51% for the former vice president.

Biden, however, has a slight edge among Democrats who identify as neither liberal nor conservative, or the more moderate wing of the party, with about 39% saying they like the former vice president compared with nearly 35% who say they like Sanders.

Polls have repeatedly suggested that Sanders has more support among young voters, who tend to lean left, than Biden or other top 2020 Democrats – including Sen. Elizabeth Warren of Massachusetts, who is running on a similar platform to her colleague from Vermont. Though he lost the Democratic nomination to former Secretary of State Hillary Clinton in 2016 by a significant margin, Sanders earned more votes from young people than Clinton and Donald Trump combined that election season.

While party loyalty may be a disqualifying issue for centrist voters, the Democratic Party appears to be moving away from that sentiment as younger generations get more involved.

Historically, older voters have participated in larger numbers than their younger counterparts, but that trend has been shifting in recent elections.

In fact, Generation Zers, millennials, and Generation Xers accounted for a narrow majority of voters in the 2018 midterms. This group – people ages 18 to 53 in 2018 – cast 62.2 million votes as compared with 60.1 million cast by baby boomers and older generations, according to Pew Research Center. Democrats took back the House in 2018, which went on to impeach Trump the following year.

Sanders’ campaign has major momentum

Older Democrats like Biden and Carville, who are both septuagenarians, have warned that Sanders (also in his 70s) is too radical to build the coalition necessary to defeat Trump. Among other things, they’ve pointed to Sanders’ lack of attachment to the Democratic Party as evidence of this. When it comes to young, progressive voters, who could play a crucial role in November, this just doesn’t appear to matter.

Electability is one of the biggest topics of conversation surrounding the 2020 Democratic primary season, with much speculation over who’s best-suited to take on and beat Trump. Along these lines, Carville last week told MSNBC: “There is only one moral imperative, in this country right now, and that is to beat Donald Trump. That’s the only moral imperative. That’s the only thing I want to hear.”

WATCH: James Carville has some thoughts about what is happening in Iowa, and across the country, for Democrats. pic.twitter.com/W9KuEeFvcO — MSNBC (@MSNBC) February 5, 2020

It’s early in the race and difficult to make precise predictions, but Sanders is on an upswing.

Sanders has been dominating his fellow 2020 Democrats in fundraising, taking in a whopping $25 million in January alone. Though the results of the disastrous Iowa caucuses remain unclear, Sanders appeared to be in a virtual tie for first with former Mayor Pete Buttigieg of South Bend, Indiana. He’s pulled ahead of Biden in some national polls, which also show him beating Trump in hypothetical head-to-head matchups, and is expected to do well Tuesday in the New Hampshire primary.

In short, Democratic critics of Sanders are running low on credible arguments against his electability as he creeps into frontrunner territory.