Des Moines, Iowa (CNN) Sen. Bernie Sanders is on a roll -- and that is precisely what worries his rivals and is rattling the Democratic establishment one week before the Iowa caucuses open the party's nominating contest.

Private concern among moderate Democrats has steadily escalated into public alarm about whether Sanders becoming the party's standard bearer would make it easier for President Donald Trump to be reelected and complicate the party's chances of winning control of the Senate and holding its majority in the House.

The heart of those concerns stem from Sanders' proud embrace of being a democratic socialist, a moniker that many worry would be a godsend for Trump and particularly hurt Democrats in swing districts the party won back in 2018.

"Bernie is surging and we're running out of time before the Iowa caucuses," Pete Buttigieg , the former mayor of South Bend, Indiana, wrote Sunday in a fundraising appeal to supporters, bluntly suggesting Sanders would be a "risk" as the party's nominee.

Minnesota Sen. Amy Klobuchar told reporters on Sunday that she believes Sanders would be bad for down ballot races in 2020 if he is the nominee.

"That's my argument. My argument is that I will make our tent bigger, our coalition wider and my coattails [are] longer. I actually have the receipts," she said. "I don't come from a state as blue as Vermont."

This is hardly a new argument -- the debate between a progressive and more moderate approach has framed the entire campaign -- but it's taken on a heightened sense of urgency amid Sanders' resilience and strength in Iowa and New Hampshire.

Sanders dismissed the criticism, telling voters on Sunday he could unify the party through his agenda and bring new excitement to the campaign to build a coalition to defeat Trump.

"Everyone says, 'Oh, Bernie is too extreme, too radical.' Not true!" Sanders said, speaking over applause in Storm Lake, Iowa.

"Our campaign is a campaign of excitement and energy."

Yet he also seemed to relish the anxiety coursing through some segments of the Democratic Party, declaring to supporters in Sioux City at his final weekend stop: "Suddenly we have the Democratic establishment very nervous about this campaign."

"If he is the nominee, he is the nominee"

In conversations with party leaders and activists, all Democrats CNN spoke to said that they would support Sanders against Trump if the Vermont senator is the nominee. But with polls in both Iowa and New Hampshire showing Sanders rising just weeks out from both contests, there is a sense of dread emanating from corners of the Democratic Party, with Republicans cheering on the Sanders surge.

Many of these Democrats believe having Sanders, a political figure who has proudly labeled himself as a democratic socialist for decades, will make it easier for Republicans to tar all Democrats running in 2020 as socialists, especially in places -- like the suburbs -- where the party made significant inroads during the 2018 midterms.

"The concerns are that he will take the party too far left," former Virginia Gov. Terry McAuliffe said. "And that most Americans don't support socialism and things like that."

But, in a comment echoed by other top Democrats, McAuliffe added that he has had the same message for Democrats who express reservations about Sanders: "Get over it."

"Whoever it is, the biggest thing for us is beating Donald Trump, and whoever the nominee is, we are going to get behind that person," McAuliffe said. "If he is the nominee, he is the nominee."

These concerns are not new for Democrats. When Sanders ran against Hillary Clinton in 2016, establishment Democrats publicly said the Vermont senator would be a liability for the party in the general election. But because the Democratic nomination fight was effectively a two-person race, Clinton and her allies were able to eventually defeat the insurgent Sanders.

The 2020 primary is an entirely different calculation and the concerns from Democrats have followed Sanders' surge in the polls.

A recent New York Times/Siena College poll found Sanders at 25% in Iowa, ahead of Buttigieg at 18%, Biden at 17% and Warren at 15%. That comes on the heels of a CNN/University of New Hampshire poll of likely Democratic primary voters found Sanders leading the race at 25%, with former Biden at 16%, Buttigieg at 15% and Warren at 12%.

A senior Democratic strategist, who requested anonymity to speak frankly about Sanders, put a finer point on the concerns: "The question is whether Bernie can help down ballot candidates sell his idea of significant tax increases and government run health care to people who already have health insurance and are hostile to tax increases."

Sanders is pledging, as president, to push " Medicare for All ," a sweeping government overhaul of the American health care system, and to make public college and universities free for all Americans and cancel all student debt.

The issue, in the minds of Democrats, is that many of the voters who gravitated towards the party in 2018 are not traditional part members and could be turned off by these sort of government programs.

"These are not traditional 'Democrats,' they are anti-Trump GOP refugees," the strategist said. "I worry they will see Trump as the devil they know and Sanders as the devil they don't."

At a campaign stop for Joe Biden on Sunday, Iowa Rep. Cindy Axne, who won a Republican-held seat in 2018, spoke to voters of the importance of down-ballot races, in a possible warning against a Sanders nomination.

"We know that we need a leader at the top of the ticket who won't just help themselves get elected," Axne said, "but will help us grow and protect the majority in the House and also make sure that we flip seats in the Senate and take back the Senate once again."

Two hours away in Storm Lake, a far different case was being made by another freshman Democratic congresswoman.

Rep. Alexandria Ocasio-Cortez, who was traveling with Sanders, said the country's challenges require bold thinking. She pushed back on the idea that Sanders could not help lead the ticket.

"Sometimes people even call Bernie divisive," Ocasio-Cortez told voters. "I don't think he's divisive, I think he distinguishes himself."

GOP hopes for ideological battle

But Republican operatives -- both privately and publicly -- have been enjoying Sanders' rise. Trump's campaign has looked to elevate Sanders in a series of statements, fundraising emails and tweets, and Republican operatives working on Senate and House races tell CNN they see Sanders atop the ticket as their best chance to reverse the gains Democrats made in 2018.

"The ideal scenario for us is one where the election becomes an ideological battle in the suburbs," said a senior Republican strategist. "Someone like Sanders or Warren really sets up the map best for us because it would have substantial down line effects in the House."

Dan Conston, the president of the Congressional Leadership Fund, a super PAC tied to Republican leadership in the House, said, "A fight over whether the country should head in the direction of socialism is especially effective in the suburbs. Socialism is about radically changing the way people's lives are now."

Jeff Weaver, a senior adviser to the Sanders campaign, dismissed the handwringing from the Democratic establishment.

"Washington, DC, is populated with consultants, pundits and insiders whose whole financial and personal self-worth is built around maintaining the status quo," Weaver said. "And the truth of the matter is that people outside of that Washington bubble want real change."

He added: "It would be a mistake to take political advice from the very group of people who created the situation that allowed Donald Trump to win."

Sanders' strength has also caught the eye of the senator's opponents, who have seized on his general election electability as an issue in the primary.

"Bernie performs the worst against Trump amongst all major candidates," Hari Sevugan, Buttigieg's deputy campaign manager, wrote in a fundraising email on Saturday, pointing to the fact that 42% of general election voters in Iowa said they would back Sanders, compared to 48% for Trump. "In short, we risk nominating a candidate who cannot beat Donald Trump in November. And that's a risk we can't take.

Rep. Dave Loebsack, a retiring congressman from Iowa who is backing Buttigieg, made this point more directly in an interview with CNN.

"I will support him, obviously, and I will do everything I can to make sure he wins," he said. "But I do believe that those who are from the industrial Midwest, those who reflects the views and the values in my part of the world are best suited to win in the general election."