(CNN) He has been dismissed, shrugged off, and, most gratingly to his campaign and supporters, ignored for long stretches by other leading primary rivals and national political pundits.

But with less than a month before the Iowa caucuses, this volatile 2020 presidential primary has taken yet another eye-popping turn: Bernie Sanders is being recognized by the Democratic establishment, sometimes in puzzlement and others in fear, as the real deal -- a legitimate contender, even a front-runner, to become the party's next presidential nominee.

It happened slowly, then, as the year turned, seemingly all at once. Maybe it was his fourth quarter fundraising total -- $34.5 million, more than anyone else and a personal best -- or the recent early-state polls, which suggest a Sanders sweep of Iowa and New Hampshire is very much in play. The 78-year-old democratic socialist always had the firmest voter base. He has not made any meaningful adjustments to his campaign strategy. What's changed is that he is beginning to show concrete signs of expanding his support in a way that few outside his loyal circles predicted.

Now, with the potential for victory in sight, Sanders has begun to sharpen his case against Joe Biden , who has mostly sought to sidestep the substance of that criticism. How much longer the former vice president can afford to avoid the kind of confrontation Sanders is seeking is an open question. Tuesday night's debate in Des Moines will mark the field's final joint exercise before the caucuses.

In recent interviews, Sanders has taken aim at Biden's core strength -- perceived electability -- by highlighting his past votes to authorize the Iraq War and, with a special nod to how it would "play" in the industrial Midwest, the North American Free Trade Agreement.

"If we're going to beat Trump, we need turnout," Sanders told CNN's Anderson Cooper on Monday night. "And to get turnout, you need energy and excitement. And I don't think that that kind of record is going to bring forth the energy we need to defeat Trump."

Sanders campaign manager Faiz Shakir expressed some exasperation at the tone of Biden's responses so far, which have, as Shakir described them, been limited to "a brush-off smile or coy comment."

Biden in a recent interview on NBC News, pushed back -- in vague terms -- on the suggestion he didn't have the juice to fire up Democratic voters .

"Bernie, I'll see you at the caucus," Biden said. "I'll see you in New Hampshire, I'll see you in the primaries, I mean, let the voters decide that, whether there's enough energy and I -- all I know is out on the stump, things are feeling really good, there's a whole lot of energy."

Sleeping on Sanders

In past gatherings, Biden and other moderates like Pete Buttigieg and Amy Klobuchar have mostly passed up potential flashpoints with Sanders, preferring to zero in on his progressive ally Elizabeth Warren, whose headline-grabbing surge in the polls over the summer made her a more appealing target.

Both Biden and Klobuchar may have unwittingly boosted Sanders in their zeal to undermine Kamala Harris and Warren, respectively. In separate venues, both described Sanders as "honest" in order to knock down other rivals during exchanges over the nuts and bolts of "Medicare for All."

Harris has since dropped out of the race and Warren is slumping after a sustained hammering over the details of her plans. Sanders, meanwhile, has largely been spared that level of scrutiny -- at least from the other campaigns -- and forged ahead with his efforts to build the diverse, working class coalition of support, bolstered by a massive, volunteer-driven organizing apparatus, his team believes will ultimately carry him to the nomination.

If Sanders succeeds, a Democratic establishment that either opposes his policies on a substantive level or views his leftist politics as a general election loser -- or some combination of the two -- will be left asking itself why it spent long stretches of the campaign treating the Vermont senator as an afterthought.

Progressive strategist Rebecca Katz, a former aide to Harry Reid in the Senate who has not endorsed a primary candidate, puts it down to the cloistered nature of high-level politics.

"The issue here is that because you have so many folks in the establishment who don't know anyone voting for Bernie, they assume that there is no one voting for Bernie," Katz said. "Bernie always had the highest floor, he always had the most amount of people who believed in him. Those people now are starting to convince their friends and family that they should be for Bernie, too."

Sanders' uptick has not gone unnoticed by Republicans. On Wednesday, Trump's campaign, which has for the most part stuck to either criticizing Biden or the Democratic field more generally, launched a rare direct attack on Sanders. In an email to reporters, the Trump team singled out his response to the killing of top Iranian general Qasem Soleimani as insufficiently bellicose, calling it evidence Sanders "cannot be trusted to protect American lives."

If that statement is the beginning of a trend, then Democrats already skeptical of Sanders will see their prevailing hypothesis -- that he cannot withstand the opposition ringer -- tested as the primary season finally kicks off.

"I think there are a number of people who are concerned that Bernie Sanders has not been fully vetted in this current field the way that other candidates have," Zac Petkanas, a Democratic strategist and former rapid response director for Hillary Clinton in 2016. "And there are a lot of unknown about him. He may have a glass jaw and will crumble the moment that the right wing attack machine turns his focus on him."

The Republican "attack machine" might be late coming to Sanders, but it has relentlessly sought during her first year in office to make Alexandria Ocasio-Cortez the bête noire of the Trumpist movement. But those efforts have done little to dim her star with progressive Democrats. Ocasio-Cortez's endorsement of Sanders, which was announced shortly after he returned to the trail following a heart attack on the first day in October, provided his campaign with a jolt -- and helped set it off on an upswing that has carried into the new year.

More than a nomination at stake

"I don't want to place too much on this, but a lot of people thought that when he had his heart attack, that was the end of his campaign," said Sara Nelson, president of the Association of Flight Attendants-CWA, which has not yet decided whether it will endorse in the primary. "And what it did was it rejuvenated it. I mean, it made him a 'real person.' People can't just talk about him as a grumpy old man. Then (the campaign) started actually showing him doing physical activity because that's what people were interested in."

The Ocasio-Cortez endorsement, which Shakir called "a moment of great courage on her part," offered Sanders a level of public validation, especially with Latinos and voters of color, he never quite achieved in 2016. It also underscored the stakes of the coming primary, which will stretch beyond this year and the next. If Sanders succeeds, an entire generation of Democrats, dating back at least to the early 1990s, will be forced to reconsider their place in the party -- a prospect in which most of his supporters would surely delight.

Still, most voters don't think in those sweeping, historical terms. Polls have routinely suggested that, as much as Medicare for All or free public college play on their minds, it is the more elemental question of trustworthiness that has consistently undergirded Sanders' support. In the final accounting, Sanders, by changing very little of himself, might very well change everything in Democratic politics.

Nabilah Islam, who is running on a platform including Medicare for All in Georgia's 7th House district primary, will be watching the first round of presidential voting with interest.

"If (Sanders) starts winning these early primaries, I think people will start realizing that his policies are resonating," Islam, who worked for Hillary Clinton in 2016, told CNN. "It's funny how the media tried to cast him as just too, too progressive. But the policies that he's advocating for are the ones that we've been looking for."

The contrast between Sanders and the top tier of the Democratic field, even as it has moved left over the past few years, played out in miniature in the aftermath of the recent US strike in Iraq. When most candidates initially responded by couching their condemnations of the attack with reminders of Soleimani's own bloody rap sheet, Sanders skipped straight to warning against a military escalation and calling for congressional action to arrest any push for war.

It was, simply, exactly what you'd expect from Sanders.

"He's been consistent on every single issue, not just for this last campaign, not just for this last year, but for his entire career," Katz said. "There's no caveats. He's in the position he is because everybody knows what he says is what he believes, because he's been saying it for decades."