Democratic presidential candidate Hillary Clinton speaks during a campaign event at Iowa State University in Ames on Jan. 12. (Patrick Semansky/AP)

Some leading Democrats are increasingly anxious about Hillary Clinton’s prospects for winning the party’s presidential nomination, warning that Sen. Bernie Sanders’s growing strength in early battleground states and strong fundraising point to a campaign that could last well into the spring.

What seemed recently to be a race largely controlled by Clinton has turned into a neck-and-neck contest with voting set to begin in less than three weeks.

On Capitol Hill and in state party headquarters, some Democrats worry that a Sanders nomination could imperil candidates down the ballot in swing districts and states. Others sense deja vu from 2008, when Clinton’s overwhelming edge cratered in the days before the Iowa caucuses.

Just as Barack Obama’s stunning upset there helped assure Democrats in later states that a black man could win votes from whites and propelled him to victory in South Carolina and other places, so, too, could a Sanders victory on Feb. 1 in Iowa and then Feb. 9 in New Hampshire ease doubts about the viability of a self-described “democratic socialist,” some said.

“It’s just like the weak spot for Barack Obama was his skin color, but he got cured of that in Iowa,” said Rep. James E. Clyburn (D-S.C.), the party’s leading African American in Congress.

Democratic presidential candidate Sen. Bernie Sanders (Vt.) speaks during a campaign stop Jan. 14 at Dartmouth College in Hanover, N.H. (John Minchillo/AP)

“If [Sanders] comes out of Iowa and New Hampshire with big victories — if it’s close in both places, that’s one thing — but if he comes out of there with big victories, hey, man, it could very well be a new day,” Clyburn added.

One Clinton ally on Capitol Hill said some in the party are starting to seriously consider what it would mean for Democrats nationally if Sanders were to win.

“There’s definitely an elevated concern expressed in the cloakroom and members-only elevators, and other places, about the impact of a Sanders nomination on congressional candidates,” Rep. Steve Israel (D-N.Y.) said.

Israel, a former chairman of the Democrats’ House campaign committee, said that a Sanders nomination “increases the level of anxiety that many of our candidates have in swing districts, where a Hillary Clinton nomination erases that anxiety.”

Sensing the tightening race, some state party officials have gone out of their way to keep the peace with supporters of Sanders, hoping to tap their energy and keep them activated for the general election campaign.

The reevaluation of the Democratic primaries — which seemed destined for a Clinton coronation after she recovered from a summer slide amid controversy over her use of a private email system while secretary of state — comes as state and national surveys show her sliding fast once again.

A Des Moines Register survey of likely Iowa caucus voters released Thursday showed a statistical dead heat, with Clinton at 42 percent and Sanders at 40. That marks a significant shift from a month ago, when Clinton held a lead of nine percentage points and saw her share of the vote at 48 percent. In New Hampshire, Sanders holds a commanding lead, 53 percent to 39 percent, according to a Monmouth University poll released this week.

Clinton and Sanders have escalated their attacks on each other, with each claiming to be the strongest general election candidate.

The new dynamic will be on display in South Carolina this weekend, when the Democratic candidates attend a party dinner and then a fish fry hosted by Clyburn ahead of their debate Sunday night. The pre-debate events, expected to draw hundreds of activists, will serve as a chance for Sanders to prove that his campaign has an effective organization beyond the first two states.

“We’re really at the front end of the process for states beyond Iowa and New Hampshire,” said Sanders adviser Tad Devine. “Part of the process is to convince people Bernie is a serious option, and doing well in early states helps.”

Clinton’s allies have said that they have always planned for a difficult primary season and that they expect their well-structured campaign to pay dividends when the race moves on to larger states with more diverse electorates than the two earliest states. They note that a recent trip to Oklahoma, part of the Super Tuesday bloc of 10 states on March 1, demonstrated their campaign’s long view of the race.

“From Day One, we have told everyone who will listen this would be a dogfight,” said Jerry Crawford, a longtime Clinton supporter in Iowa. “Hillary will continue to fight for every vote just as she has done since Day One in Iowa, and I wouldn’t trade places with any other campaign.”

Whether or not he wins, Sanders’s rise has created challenges for party leaders by highlighting policy differences between the Democratic establishment and the party’s support base.

Many Sanders proposals — Medicare for all, free college and breaking up big banks — go beyond congressional Democrats’ agenda but are embraced by an ascendant wing of the party.

Those policy prescriptions win support in primaries, but many Democratic elites fear how they would play in a general election. At the same time, Democratic leaders know they can’t afford to alienate an energized party base.

Some recent surveys suggest that Sanders is drawing support beyond the liberals and young voters who have flocked to his rallies.

A Quinnipiac University poll early this month found Sanders trailing Clinton by an insignificant two percentage points among moderate and conservative Democrats, a sharp shift from Clinton’s 24 percentage-point lead among that group in December.

“Whatever the success that Senator Sanders, that Bernie Sanders, has, I think it’s important to recognize that his supporters are essential to our success in winning the White House,” House Minority Leader Nancy Pelosi (Calif.) told reporters in the Capitol on Wednesday.

In the Senate, more than two-thirds of the Democratic caucus has endorsed Clinton. For now, the senators will remain calm, even if she loses the first two states, according to a senior consultant working on Senate races.

However, full-fledged panic would set in if Clinton loses the Nevada caucuses, wedged in between New Hampshire and South Carolina, the consultant said.

A Clinton defeat would complicate matters for one of the country’s most vulnerable Democrats, Rep. Cheri Bustos (D-Ill.). Bustos said that much of her campaign strategy is based on energizing female voters with the potential of a female presidential nominee . “There’s a lot of excitement about having a woman at the top of the ticket,” Bustos said, without directly critiquing Sanders.

David Pepper, the Ohio Democratic Party chairman, said Clinton’s infrastructure remains very strong after her decisive victory over Obama there eight years ago.

But, he said, the Sanders team has been on the move. Pepper said he allowed Sanders’s supporters to use party headquarters to host a national “meet-up,” and Pepper met with Sanders after 6,000 attended a rally in Cleveland.

One of Sanders’s supporters recently announced a challenge to Rep. James B. Renacci (R-Ohio), a third-term Republican whose most recent reelection drew little competition.

“Our strategy of being intensely neutral and welcoming to all is paying off,” Pepper said.

Pepper’s approach differs from that of the Democratic National Committee, whose leadership has been feuding with the Sanders campaign over debate scheduling and other areas where Sanders allies say the party has shown favoritism to Clinton.

The central fight among Democrats could come down to how voters will perceive Sanders, the wispy-haired 74-year-old former mayor of Burlington, Vt., and whether they think his ideology could be a help or a hindrance.

“I’m deeply concerned that in November swing voters are not going to vote for a socialist,” said Israel, who is retiring.

Clyburn, however, said he wasn’t convinced Sanders’s ideology would be a drag, at least not in the primaries. He credited the senator for his consistent delivery of an agenda focused on erasing income inequality.

“I’m out there, and I know what Democrats are feeling,” he said. “Democrats really feel strongly about this income-inequality business. That is a big, big issue.”

Anne Gearan, John Wagner and Scott Clement contributed to this report.