(CNN) Bernie Sanders, the clear front-runner for the Democratic presidential nomination after his commanding victory in Nevada on Saturday, often says the principal reason he can beat President Donald Trump in a general election is that he will massively increase voter turnout.

But very few Democratic strategists, from across the ideological spectrum, agree.

Sanders might find a way to win a general election if he secures the nomination. But few serious analysts in the Democratic Party believe he could win the way he says, by burying Trump under a tidal wave of new liberal voters.

The idea that the senator from Vermont can win by expanding the electorate "is the equivalent of climate denial," longtime Democratic strategist James Carville told me. "If you believe that, you are dumb as a climate denier. Not only is there no evidence to support your hypothesis; there is all the evidence in the world to refute it."

Carville has emerged as a leader among Democrats concerned that nominating Sanders will doom the party to defeat against Trump and put the House majority at grave risk as well. Unlike Carville, Sean McElwee, founder of the liberal-leaning group Data for Progress, believes Sanders can find a pathway to victory against Trump by attracting working-class voters across racial lines. But McElwee agrees with Carville that no candidate, Sanders included, can bet on winning mostly by transforming the nature of who votes.

"I think that all campaigns are incentivized to portray themselves as doing something unique and groundbreaking and really changing the structure of turnout." McElwee says. "But turnout is a pretty durable attribute and it tends to correlate with intrinsic human identities: Older people tend to vote at much higher rates than younger; college educated vote more than non-; homeowners vote more than renters. It is really, really hard using the tools available to campaigns to change that."

This dispute has profound implications as Democrats' assess Sanders' potential viability as a general election candidate. The Democratic front-runner brushes off concerns about whether his agenda will alienate swing voters by insisting he can compensate by bringing in millions of new voters to overwhelm them.

But the evidence strongly suggests that's wrong, and that to win Sanders would, in fact, need to persuade swing voters, just like any other potential nominee. Democratic experts both sympathetic to Sanders and skeptical of him agree the real question is whether he can do that -- not whether he can count on a massive cavalry of new voters to tilt the election.

The idea that a turnout surge will obviate the need to convince swing voters "is magical pixie dust that he and others like him wave over their candidacies to explain away any problems," says Ruy Teixeira, a leading Democratic expert on the electorate and a senior fellow at the liberal Center for American Progress.

Nonvoters are no monolith

Expanding turnout is the core of the electability argument that Sanders offers in his campaign appearances. "At last week's contentious debate in Nevada , he said, "In order to beat Donald Trump, we're going to need the largest voter turnout in the history of the United States," and at Tuesday night's debate in South Carolina he promised to keep working toward that historic turnout. "We need to bring working people back into the Democratic Party," he said. "We need to get young people voting in a way that they never have before. That is what our campaign is about."

During a campaign rally in Iowa last month, Sanders argued at length that he's best suited among the Democratic candidates to inspire such a surge in voting. "So I would hope the people of Iowa, people of America, ask themselves: Which campaign is the campaign of energy and excitement?" he insisted in Des Moines on Martin Luther King Jr. Day. "Which is the campaign that is reaching out to working class people, many of whom are disillusioned with the political process and too often do not vote? ... [The] simple truth is that young people have not voted at the kind of rate that they should have. Which campaign is capable of bringing in millions of young people into the political process?"

For the Democrats skeptical of that argument, the problem isn't the assumption that more people will vote in 2020 than in 2016. Experts in both parties believe that as many as 15 million to 20 million more people could turn out for this presidential race than the 139 million who voted in 2016. If those predictions bear out, it would produce the highest turnout among eligible voters since 1908 , which was before women had the right to vote.

Rather, to the skeptics, the flaw in Sanders' argument is his claim that he can expand turnout in a way that provides a decisive advantage to the Democratic nominee alone. Experts on the electorate in both parties, as well as political scientists who study voting patterns, almost universally agree that at this point the massive pool of nonvoters does not break decisively toward one party.

"This is why the whole Sanders theory of the case is so suspect," says Teixeira. "It's not like we don't know about nonvoters ... and, given what we know, it's not at all clear that his case makes much sense."

An exhaustive study from the Knight Foundation that examined the roughly 100 million eligible Americans who did not vote in 2016 underscores Teixeira's point. For the study, which was released last week, the foundation commissioned a survey of 12,000 nonvoters nationwide and in swing states, and held focus groups with Americans who habitually do not vote. The results found nonvoters united by their disconnection from the political process and disengagement from the news, but divided quite closely in their views of the two parties. In the survey, for instance, nonvoters expressed comparably negative views about both political parties. Asked who they might support if they voted in 2020, they split roughly evenly, with about one-third picking an unnamed Democrat, slightly fewer picking Trump and the rest saying they would vote for a third-party candidate or were unsure.

"The problem with the conventional wisdom is it tends to caricature who is a nonvoter," says Sam Gill, Knight's senior vice president. "I think it's wrong to think that more turnout therefore means a Democratic win. There's nothing in this poll that suggests that should be the obvious [conclusion]."

The pool of potential new voters in 2020 might lean slightly more Democratic than Knight's overall poll results found. Its survey found that about three-fifths of nonvoters were significantly more engaged with the news and politics than the remainder; that more engaged group was also more skeptical of Trump. And the poll surveyed only nonvoters 25 and older, rather than young people who are newly eligible to vote. In a separate survey Knight conducted, those young people were also more skeptical of Trump than the total pool of nonvoters. Given the preponderant support Sanders has attracted from younger voters this year and in his 2016 run, he almost certainly is the potential Democratic nominee best suited to maximize turnout from those emerging generations.

But the geographic distribution of nonvoters creates challenges for a Democratic strategy centered on mobilizing them, especially in the Trump era. On a national basis, the best evidence suggests, the Americans who are eligible to vote but don't split about equally between whites without college degrees, who lean Republican, on one side; and minorities and college-educated whites, who lean Democratic, on the other.

But the distribution looks very different in the Rust Belt states that tilted the 2016 election. In Wisconsin, Michigan and Pennsylvania, the three states Trump dislodged from the "blue wall," whites without college degrees represented a clear majority of the adults who were eligible to vote but did not, according to calculations from census data by David Wasserman of The Cook Political Report. The adults who have become newly eligible to vote in those states since 2016, mostly by turning 18, do lean more toward minorities, according to analysis by the States of Change project , which Teixeira directs. But even accounting for those young entrants into the electorate, many Democrats believe that Trump has a bigger universe of potential new voters available to harvest across the Rust Belt than the Democratic nominee does. (Arizona, in the Sun Belt, may be the battleground state where a mobilization strategy makes the most sense for Democrats: There, nonwhites significantly outnumber noncollege whites as a share of eligible voters, according to Wasserman's calculations.)

Other side may mobilize too

Another fundamental challenge looms over a strategy built on winning through mobilization: Any candidate, like Sanders, with a persona strong enough to excite positive turnout on his side risks igniting "negative" turnout on the other. Many Democrats believe that the unrelenting partisan conflict of the Trump presidency ensures that their supporters will turn out in large numbers against him in November. If the party picks a polarizing nominee of its own, they fear, it will make it easier for Trump to generate massive turnout from the Republican base against the Democrat. Even if Democrats want to pursue a strategy revolving around mobilization, Teixeira argues, "You can argue it's possible that Sanders is the worst possible candidate to do it. He's so polarizing. He's likely to jack up turnout on the other side at least as much."

If Sanders can't win a general election by changing the electorate, as these Democratic experts believe, that means he, like any other potential nominee, would need to win primarily by converting swing voters. Though Sanders always stresses mobilization, especially of young people, some of his supporters -- and advisers -- believe that he would be more likely to beat Trump by attracting working-class voters across racial lines, including whites, African Americans and Hispanics.

"If you are hitching your wagon on a youth quake [of new voters] you are in a bad place," says McElwee. "But Bernie doesn't have to hitch his argument on that. Bernie has a persuasion argument for swing voters."

McElwee argues that Sanders can win back some working-class white voters who backed Trump in 2016 -- as well as prevent possible defection from working-class African Americans and Hispanics drawn to Trump's economic record -- by arguing that Trump has a record of "broken promises for workers" on issues such as health care and taxes.

Ben Tulchin, Sanders' campaign pollster, has also long argued that Sanders is better positioned than any other Democrat to beat Trump by building a coalition centered on working-class voters across racial lines.

"You have to give voters a reason to fire the incumbent president," Tulchin said in an interview. "Our case is Trump is a product of the rigged economy, he doesn't pay his workers fairly and he doesn't pay his fair share of taxes -- he takes that money and buys himself the presidency to give himself and his billionaire cronies a tax cut while trying to take away health care from working people. Bernie can say I'm going to stand up for you: better wages, health care, lower prescription drug costs. And if we can prosecute that case -- what we see [is gains] with more downscale independent voters -- who get pissed off by corrupt politicians."

But even that potential blue-collar path for Sanders contains two enormous gambles. One is that he can add enough working-class voters to offset what almost all Democrats expect would be substantial resistance to his unprecedented tax-and-spending plans among the college-educated suburbanites who moved toward the party in 2018 because of their distaste for Trump. (A recent analysis using 170,000 interviews from the nonpartisan Nationscape survey found that Joe Biden and Sanders posted similar leads over Trump overall in tests of 2020 sentiment, but that the former vice president ran much better among college-educated white voters.)

The other bet is that Sanders could build an economic connection with blue-collar voters strong enough to overcome their resistance to many of his views on issues relating to race and culture. Polls last year by the Marist Institute found that most noncollege whites supported such core Sanders economic proposals as a wealth tax on large fortunes and raising the minimum wage. But they registered overwhelming opposition to other ideas he's embraced: In one Marist survey, 67% of noncollege whites opposed eliminating the death penalty, 72% opposed decriminalizing illegal border crossing and 76% rejected providing subsidized health care to undocumented immigrants. In the Marist polling, a majority of noncollege whites have also consistently opposed one of Sanders' core policy proposals: a single-payer national health care system that would eliminate private insurance with only a very few exceptions.

The fear among the Democrats dubious of the senator is that while polls now generally show him running about as well against Trump as Biden does, when more voters learn about the specifics of Sanders' agenda -- not to mention his self-identification as a "democratic socialist" -- his support will tumble.

"You can be for Bernie for any reason you want ... but please do not bore me with the argument that you have a key to electoral success," says Carville, who ran Bill Clinton's winning 1992 campaign for the presidency. "There's no evidence. And, in fact, all evidence is massively to the contrary. "

Tulchin counters that voters will overlook disagreements with any individual aspect of Sanders' agenda if they believe he is fighting for their economic interests.

"Yes, they are going to come after us. But it's not like we just showed up yesterday and voters don't know who we are," he says. "You can successfully motivate working-class voters on economic issues if you deliver a persuasive economic message."

But for the many Democrats -- especially but not exclusively in the party's moderate wing -- who consider Sanders unelectable, all of these arguments from the senator's camp represent far too great a gamble when the consequence of failure would be a second term for Trump.