Democratic presidential candidate Hillary Clinton speaks at an event in Davenport, Iowa, on Monday during a campaign swing through the state. (Daniel Acker/Bloomberg News)

Since the start of Hillary Clinton’s second campaign for president, the Iowa caucuses have been a crucial test of her rebound attempt and the lessons learned from defeat here in 2008.

With less than a month to go, the Democrat is holding a steady lead in Iowa over rival Bernie Sanders. The question facing the campaign now is whether a narrow win is good enough — or whether Clinton needs a blowout to avoid looking weak against an improbable far-left competitor.

“I know that if I get off to a good start here in Iowa, we’re halfway home,” Clinton said here this week at the start of a two-day, six-event swing from one side of the state to the other. It was her 75th appearance in Iowa since making it her first stop after announcing her candidacy in April, her campaign said.

The battle for Iowa is mostly a battle of expectations, which for Clinton means a victory that cements her status as the national Democratic front-runner and dispels the ghosts of 2008. Expectations are lower for Sanders, a senator from Vermont who at times sounds almost surprised by his success.

In an interview last week, Sanders pointed to crowd sizes during his ongoing trip through the eastern side of the state: 600 in Muscatine, 1,850 in Davenport, 500 in Keokuk, 800 in Ottumwa and 500 in Knoxville.

Democratic presidential candidate Bernie Sanders addresses the crowd at a town hall event in Muscatine, Iowa, on Dec. 29. (Mark Kauzlarich/Reuters)

“It makes me think we have some momentum that can carry us over the top here in Iowa,” he said.

In Des Moines on Monday, Clinton called the state the “first line of defense” against Republican ambitions.

“That is a solemn responsibility that the people of Iowa hold in their hands,” she told an enthusiastic crowd of about 600. “So please, join my campaign. Be part of the vision and the values that I believe are in the best interest of our country and reflect the better angels of ourselves.”

Clinton has invested enormous resources in Iowa, with 26 offices and a paid staff far larger than that of any other candidate on either side of the race. The campaign claimed at least one committed supporter in each of the state’s 1,681 precincts by early last summer. The infrastructure serves a practical and symbolic function, flooding the state with organizing muscle to turn out committed supporters Feb. 1 and demonstrating that Clinton learned the lessons of overconfidence and hubris.

“It should be clear to anyone watching that Hillary is fighting for every vote in Iowa and taking nothing for granted,” said longtime Iowa supporter Jerry Crawford. “Our organizers are out every day picking up supporter cards. I believe this is the best organization ever created in Iowa.”

By most metrics, the investment has paid off. Clinton leads Sanders by five to 18 points in Iowa polls taken in December and compiled by Real Clear Politics. Her strength among establishment Democrats in the state was one factor that scared off a potential challenge from Vice President Biden, who announced in October that he wouldn’t run.

But Iowa Democrats can be a contrarian lot, and the labor-intensive caucuses are notoriously unpredictable. Sanders makes the 2016 caucuses even more of a wild card. He is trying to bring in a large number of people who normally don’t participate — the kind of people who have contributed to his big rallies.

From an office tucked between a supermarket and a liquor store in Des Moines, Sanders’s brain trust in Iowa is plotting ways to broaden the electorate.

The focus has been on several demographics more favorable to the senator, including people younger than 30, particularly college students; working-class families who could be particularly receptive to Sanders’s message of economic justice; and older progressives. Aides joke that they have a lock on the men-with-ponytails subset.

Behind the strategy is a calculation that Clinton’s strength is also her weakness — that her establishment credentials, fundraising prowess and mainstream appeal do not translate to energized, committed voters. Sanders supporters note that her events are smaller, shorter and more controlled than his.

The caucuses are not a straightforward win-loss proposition, said Dennis Goldford, a political science professor at Drake University. Sometimes the real story is the second- or third-place finisher because of what the showing says about that candidate’s strength, the numerical winner’s weaknesses or both. Think No. 3 finisher Howard Dean in 2004.

“It’s more like judging gymnastics than basketball,” Goldford said.

He said that by the subjective measures of Iowa, Clinton should win decisively in order to put to rest doubts about the candidate and the kind of campaign she’s running this time. He argues that Clinton must increase her current support, which stands at 48 percent to 52 percent, to be judged the clear victor.

“If she wins in the upper 50s or over, that’s a good, solid win for her. If he holds her to the low 50s, that’s an embarrassment for her, and it will leave her politically bleeding,” Goldford said.

Another uncertainty in the race is former Maryland governor Martin O’Malley, whose supporters will be up for grabs if he does not meet the 15 percent threshold required in many caucus precincts.

Sanders spent three days in the state last week, wrapping up his trip with a New Year’s Eve party that drew hundreds of people to a pair of ballrooms in a downtown Des Moines hotel. He is scheduled to return Friday.

At stops in mostly small towns last week, Sanders said he will prevail in Iowa if the turnout is higher than usual on Feb. 1. If it is lower, he has acknowledged, he probably will lose to Clinton.

“We have, I believe, a good chance to win in Iowa,” Sanders said Monday during an appearance in Manchester, N.H., at a youth summit. “If we can win in Iowa and here in New Hampshire, I believe we have a path toward victory.”

In 2008, the last time there was a competitive Democratic contest, almost 240,000 people turned out — nearly double the more typical participation of 124,000 four years earlier. Much of the swell was attributed to enthusiasm about Barack Obama’s candidacy, something the Sanders campaign says it is trying to replicate.

Many of his Iowa stops have been in college towns, an obvious attempt to stoke turnout among a demographic that leans heavily toward the 74-year-old senator.

A Des Moines Register-Bloomberg News poll last month showed Clinton with a nine-percentage-point lead among all likely Democratic caucus-goers in Iowa. But among voters younger than 45, Sanders led Clinton by 27 percentage points. Meanwhile, among those 65 and older, Clinton’s advantage was 40 percentage points.

At a Clinton event here in Davenport on Monday, a midday audience of 400 was enthusiastic — especially when the candidate threw red meat about Republicans — but tame in comparison to the screaming, jam-packed crowds Sanders regularly attracts. The Clinton campaign billed it as an organizing outreach with a mix of declared supporters and undecided voters.

Kay Ungurean of Davenport has been to just two candidate events this cycle — one for Sanders and one for Clinton. She is clearly torn.

“I’m listening and paying attention,” Ungurean said. Her decision will be based in large part on which candidate she thinks can defeat the Republican nominee in November, she said, “and who would be better at working with Congress — or who’s the candidate that Congress would work with, how about that.”

It is widely assumed that Clinton will come out ahead on both those measures, but Ungurean is not completely sold.

“If she’s the candidate, I want him to be her running mate,” she said.

Wagner reported from Manchester, N.H.