Former secretary of state Hillary Clinton says she's relieved as the votes from the Iowa Caucus continue to pour in, and she's excited to continue debating with Sen. Bernie Sanders (I-Vt.) about the Democratic Party's ideals. (Reuters)

Hillary Clinton’s hair’s-breadth win over Bernie Sanders in the first contest of the Democratic nominating season has sent a loud message: Every day from now until November is going to be a battle.

That realization was settling in Tuesday with a party hierarchy that has united almost unanimously behind her candidacy — many on the assumption that the former secretary of state had practically a clear shot at becoming the Democratic standard-bearer.

But the narrowness of Clinton’s margin over the Vermont senator, who had been trailing by 30 points in polls as recently as November, came as a surprise to many of her backers. Indeed, had it not been for the superiority of her ground operation in Iowa, many say privately that she would have been swamped by the wave of enthusiasm for Sanders.

After campaigning on her behalf in Iowa, “I was thinking it’s going to be like a point, or two. Not, you know, 0.3 or 0.4,” said Sen. Timothy M. Kaine (D-Va.), who is regarded as a potential vice presidential pick for Clinton.

“I think the whole thing is going to be closer all the way to November,” Kaine said.

Money is not likely to be a problem for Democratic presidential candidate Bernie Sanders. In January, the Vermont senator raised $20 million, almost all of it over the Internet in contributions that averaged about $27. (John Minchillo/AP)

Most leading Democrats, and the interest groups around them, remain confident that Clinton will emerge with the nomination.

Clinton’s highest-profile ally in Congress, Sen. Charles E. Schumer (D-N.Y.), played down Sanders’s showing in Iowa, citing the Clinton family’s tortured history with Iowa.

Bill Clinton barely competed there in the 1992 race that put him in the White House. And Hillary Clinton lost badly in Iowa when she ran in 2008 against Barack Obama.

“You know, Iowa was hardly Hillary’s best state. Last time she came in third, and she won” this time, said Schumer, who is in line to become the Senate minority leader.

However, Clinton’s battle with Sanders has exposed vulnerabilities that her backers find worrisome. Chief among them is what is being called an “enthusiasm gap” — an apparent inability to ignite the kind of excitement that the gruff, rumpled Sanders is generating among young people and on the left.

Not even the prospect of making history as the first female president has been enough to add electricity to Clinton’s candidacy.

Hillary Clinton and Bernie Sanders split the Iowa caucuses nearly evenly – but how? (Peter Stevenson/The Washington Post)

Groups such as Planned Parenthood and NARAL, an abortion rights advocacy organization, engaged heavily in the final weeks of the Iowa campaign with voters on the ground and on the state’s airwaves. Women voted with Clinton overall, but entrance poll results show that the support of these groups did little to blunt Sanders’s appeal to their core constituency: unmarried women, who skew younger and supported Sanders over Clinton by 10 points.

Iowa “was a wake-up call,” said a person involved with outside efforts on Clinton’s behalf who spoke on the condition of anonymity to talk candidly about the race.

The rules of the Democratic nominating process and the unique nature of Sanders’s candidacy, meanwhile, give him both the resources and the motivation to stay in the race, even if he loses in a string of states after his expected victory in New Hampshire on Feb. 9.

Republican battles in the later states will be winner-take-all, which means that a candidate on a hot streak would rack up delegates quickly. Democrats allocate their delegates in proportion to vote totals, which slows the process.

Nor is money likely to be a problem for Sanders, who in January raised $20 million, almost all of it over the Internet in contributions that averaged about $27. Those small donors are a renewable resource; they can be tapped again and again.

“In my view, he could run this right down to the convention,” said Harold Ickes, a longtime Clinton adviser who helped oversee her delegate operation in 2008 and who is working outside her campaign with the pro-Clinton super PAC Priorities USA Action.

Clinton backer Steve Rosenthal, a political strategist with close ties to organized labor, said of Sanders: “He’s got money. He’s got a compelling message. And he’s becoming a better candidate as this is going on.”

Kaine and other senators expressed optimism that although Sanders may stay in the race deep into the spring, she could win enough races in March to make clear she will eventually secure the nomination.

However, the longer-term challenge, according to Democratic senators, is to sharpen her pitch to voters. Clinton tends to dwell on the granularity of her policy proposals — many of which are incremental steps on a traditional Democratic agenda — at a time when voters are in a rebellious mood.

“One of the things that I know the team will grapple with — and this is something that people like me have to grapple with — is just making it simple and straightforward,” Kaine said. “Bernie’s message is pretty darn simple. And it’s a message that kind of the rich are stepping all over everybody.”

“We know she would be good at the job and the governing part of the job, and governing is complicated,” Kaine added. “But sometimes you’ve got to make the message a lot simpler than the realities of governing.”

The lack of focus in Clinton’s message to voters has emerged as a weakness. Her stump speech, which can wind on for 40 minutes or more on the minutiae of virtually every major policy detail, tends to impress voters at her events, but it poses a challenge for her surrogates, who are countering a far simpler message from Sanders focused on income inequality.

“She’s going to have to change on that,” said one ally, who spoke on the condition of anonymity to talk candidly. “She’s going to have to get more succinct on messages that are powerful, around a small subset of issues.”

“If you’re in a primary state, she’s going to have to hone it down to the top three and just bang away at it,” the person added.

It wasn’t just the college vote that boosted Sanders. He over-performed in some rural areas and some urban areas that were expected to lean toward Clinton, as well as among union members.

If there is any consolation for her allies, it is knowing that Clinton has often been at her best when she is under pressure.

“Hillary just has to be her. The Hillary that we see when we’re talking to her one-on-one or as a group is completely different than sometimes what people might see on the campaign trail, because she has to be so guarded,” said Sen. Joe Manchin III (D-W.Va.). “This is a contest now.”

Kane reported from Washington. Karoun Demirjian in Washington and Abby Phillip in New Hampshire contributed to this report.