Democrats backing Hillary Clinton, nervously eyeing Sen. Bernie Sanders' growing strength in the early nominating states, are turning to a new strategy to raise doubts about his candidacy, highlighting his socialist beliefs to warn that he would be an electoral disaster who would frighten swing voters and send Democrats in tight congressional and governor's races to defeat.

It is a scenario many Democrats long dismissed as even remotely plausible: the 74-year-old Sanders, a registered independent who self-identifies as a democratic socialist, as their nominee. But the possibility of his defeating Clinton in Iowa and New Hampshire next month has prompted some of her prominent supporters to discuss how they could attack Sanders if his candidacy began to look less like a threat and more like a runaway train: calling him unelectable and warning that Republicans would have a field day if he were the Democratic nominee.

"Here in the heartland, we like our politicians in the mainstream, and he is not — he's a socialist," said Gov. Jay Nixon of Missouri, who is term-limited and working to elect a Democratic successor. "He's entitled to his positions, and it's a big-tent party, but as far as having him at the top of the ticket, it would be a meltdown all the way down the ballot."

And after months of ignoring Republican cheerleading for Sanders, Clinton's campaign has started aggressively highlighting how much the opposition is openly providing him aid and comfort — mostly recently in a new ad by Karl Rove's group American Crossroads that echoes Sanders' attacks on Clinton's ties to Wall Street.

"Republicans and their 'super PACs' have made clear the candidate they're actually afraid to face," said Jennifer Palmieri, Clinton's communications director.

Sanders, for his part, has taken to highlighting polls that show him faring better than Clinton against some of the Republican candidates, and his aides insist that voters will look past labels to consider his record, ideas and proposals.

Yet Clinton herself has begun urging activists in Iowa to consider "electability and how we make sure we have a Democrat going back into that White House on Jan. 20, 2017." And her supporters, with the campaign's blessing, are aggressively moving to sow doubts about Sanders' viability, a tactic aimed at alarming primary voters concerned about retaining the presidency and regaining the Senate.

For all the authorized fear-mongering, though, it is clear that few Democrats seeking re-election this year relish the prospect of running with Sanders on top of the ticket.

"It wouldn't be helpful outside Vermont, Massachusetts, Berkeley, Palo Alto and Ann Arbor," said Rep. Steve Cohen, D-Tenn.

Not many Democratic leaders believe they will ultimately face such a scenario, even if Sanders did take a 27-point lead over Clinton in a CNN/WMUR New Hampshire poll released Tuesday. Yet their willingness to consider it underscores the uncertainty hanging over the race — and the growing frustration among Clinton's supporters, who say her leading rival has received little scrutiny from the news media or Republicans.

"The Republicans won't touch him because they can't wait to run an ad with a hammer and sickle," said Sen. Claire McCaskill of Missouri, a supporter of Clinton's.

Sanders and his supporters deny that he would be a drag on other Democratic candidates. And Tad Devine, Sanders' consultant, recalled another election when Clinton's supporters said her primary opponent was unelectable. "This is all so reminiscent about what they said about Obama in 2008," he said.

Instead, he framed the primary as a choice between a candidate who is inspiring younger, new and independent voters or one running a 1990s-era race, focused on suburban swing voters, seizing the middle ground on issues like taxes and pushing "incremental change."

As for the socialist label, Devine said: "Voters are going to be bombarded with a phenomenal amount of information about him, his life, his political philosophy and his plans, and they're not going to let that word stop them from looking at him and considering him."

Yet after largely ignoring Sanders as he raised tens of millions of dollars and built a robust grass-roots following, mainstream Democrats are now following Clinton's lead.

"I can tell you, as somebody who ran the Democratic Governors Association, that candidates in purple states would face serious problems with him on top of the ticket," said Gov. Jack Markell of Delaware, referring to the most politically competitive states.

"Having somebody who is identified more as a socialist in many decades of public service than as a Democrat makes it impossible for Democrats in a state like Missouri," McCaskill said of her state, which could have competitive races for governor and senator this fall. "And it makes it very difficult for Democrats in states like Ohio, Pennsylvania and Florida," she added, referring to states where some of the hardest-fought Senate races will take place.

Some Democrats are even talking openly about a kind of cautionary Democratic attack on Sanders, to show how much the party could be harmed if he were the nominee and Republicans got to sink their teeth into him.

"Some third party will say, 'This is what the first ad of the general election is going to look like,'" said James Carville, the longtime Clinton adviser, envisioning a commercial savaging Sanders for supporting tax increases and single-payer health care. "Once you get the nomination, they are not going to play nice."

After the ranks of Democrats in Congress and state capitols thinned under President Barack Obama, the party's standard-bearer is no small issue. Democrats need at least four seats to win the Senate, and their hopes are promising largely because the races will be fought in states like Wisconsin, Illinois, Nevada and Pennsylvania, which tend to vote for Democratic presidents or at least be hotly contested. They are unlikely to recapture the House, but they could pick up as many as a dozen seats under good conditions. And the outcome of at least five governor's races will be determined in part by the national political environment.

A Sanders-led ticket generates two sets of fears among Clinton supporters: that other Democratic candidates could be linked to his staunchly liberal views, particularly his call to raise taxes, even on middle-class families, to help finance his universal health care plan; and that more mainstream Democrats would have to answer to voters uneasy about what it means to be a European-style social democrat.

"Hillary Clinton doesn't have to explain socialism to suburban voters," said Rep. Steve Israel of New York, the former head of the campaign arm for House Democrats, whose hardest-fought races this year include districts outside Philadelphia, Washington and Chicago.

House Democrats got a taste of those challenges last fall. As many of their candidates met in Washington with consultants, donors and reporters, word leaked that Sanders was to give a speech explaining what it means to be a democratic socialist. "We had candidates and consultants calling us, emailing us, saying: 'What do we say about this? How do we explain this?'" recalled a House Democratic official, who requested anonymity because he was not authorized to intervene in the presidential race.

The official drafted a mock question-and-answer memo.

"Senator Sanders has caught fire in the Democratic primary. He is a democratic socialist. Are you a democratic socialist?" went one of the questions. "No," was the recommended response.

Another question asked the difference between a Democrat and a socialist. Candidates were urged to express pride in being a Democrat but also belief in capitalism and small businesses, "the engine of our economy."

To some Democrats, efforts to portray their party as extremist would be made easier were they to nominate a left-wing crusader.

"The things Republicans lied about when they attacked the president would be more true with Bernie in terms of how far left he is," Markell said.

And Matthew Miller, a former top aide at the Democratic Senatorial Campaign Committee, said Sanders would be burdened with the two most glaring vulnerabilities of Republicans Donald Trump and Sen. Ted Cruz of Texas. "He is hard to see as president, like Trump, and his politics are so far to the left, like Cruz's are to the right."

As for whether Democrats risked turning off Sanders' legion of youthful supporters by assailing his electability, McCaskill said the party would have no trouble healing.