The Democrats are going into their convention gleeful and united. Can they stay that way?

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In a U.S. presidential campaign that has had multiple twists and turns, there is one element that actually is upside down: The Democrats – a month ago in despair, now proclaiming the “joy” of their endeavour – are storming into Chicago next week for their national convention with an astonishing sense of unity.

That’s a reversal of the customary pattern. Parties usually go into their conventions with strains that need to be resolved, only to emerge united after four days of alcohol-fuelled receptions, inspiring speeches, and a valedictory celebration where the strife is buried beneath the descent from the rafters of thousands of colourful balloons. The Democrats are doing the opposite. They have repealed the laws of political physics with their togetherness preceding the convention. Instead, they face the unusual danger of fissures in the days and weeks after.

That is a real possibility, and a real threat. About 100,000 Michigan Democrats spurned Joe Biden and voted “uncommitted” in the late February Democratic primary in protest of the administration’s policies in the Israel-Hamas War. Those resentments haven’t substantially receded, and now pose a dual threat to the Democrats: they signal skepticism of Ms. Harris from the party’s progressive wing, and they are a special danger for her in the swing state of Michigan, with its large population of Israel-skeptical Arab Americans.

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The unusual phenomenon of the 2024 Democrats was prompted by what initially seemed like a crisis: Joe Biden’s withdrawal from the race after a disastrous June debate performance, and his swift replacement by Ms. Harris, who quickly rallied the party to her side. But at the heart of all of this is a mystery, the resolution of which could determine the outcome of the election.

Can a party that sometimes defines itself as a union of disunited factions – the early-20th-century Oklahoma cowboy humorist Will Rogers liked to say that “Democrats never agree on anything, that’s why they’re Democrats” – retain its sense of unity? That’s a potentially determining factor for the November election, because in a contest with so few undecided voters the result could come down to which party better mobilizes its own adherents.

The answer to that question – whether the Democrats defy or redeem the other part of the Will Rogers aphorism: “I’m not a member of any organized political party. I’m a Democrat” – will shape the other mysteries surrounding the convention.

“Unity works for a party if it is combined with enthusiasm for the candidate,” said L. Sandy Maisel, a political scientist at Colby College in Waterville, Me. “There’s evidence you’re seeing that in the Democratic Party right now. If they can retain that enthusiasm and unity, they could win.”

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The party that will gather at Chicago’s United Center on Monday is a far different party than the one that nominated Mr. Biden in the same venue four years ago.

It is a far different party than it was only a month ago, when Mr. Biden was struggling to retain his position as the presumptive Democratic nominee. And that difference is redounding to Ms. Harris’s advantage.

In a Blueprint Topline survey, the only Biden administration issue area that even a quarter of the public believes the Vice-President has been solely responsible for is abortion policy, a clear winner with the campaign’s target constituency. (Only one in seven of those surveyed thought Ms. Harris was solely responsible for the surge in crossings at the Mexican border, a vulnerability for the Democrats.)

That means Ms. Harris may be free to campaign with less of the weight of the Biden record than the Trump team believes, a huge advantage at a time when the President himself has high disapproval ratings.

The Democratic ticket still faces headwinds in some swing states, and Ms. Harris’s lead in three of them is within the polling margin of error – a fact the Vice-President seemed to emphasize in her first appearance with her running mate, Minnesota Governor Tim Walz. “To all the friends listening, we also need to level set,” she said, adding a note of sobriety to the exultation. “We are the underdogs in this race.”

There is one looming threat that the Harris team has not yet confronted: the questions she must ultimately face about the depth of her knowledge of Mr. Biden’s frailty and whether she bears some guilt for shielding that from public view.

Plus this: She has yet to make an unforced error, as all candidates do. She has yet to face Mr. Trump in a debate, as she will in less than a month. She has yet to experience the frustration, and perhaps the erosion of support, that comes after being the target of relentless, withering attacks, as she will be.

One danger sign identified in the latest New York Times/Siena College poll of voters in Wisconsin, Michigan and Pennsylvania, all key swing states: While 37 per cent of the voters surveyed said in October that Mr. Biden was too liberal, 42 per cent expressed that view of Ms. Harris this month.

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Even so, the sense of desperation that followed Mr. Biden’s debate performance has lifted.

The polls that showed a freshet, and then a flood, of support for Donald Trump have taken a hairpin turn in Ms. Harris’s direction. A rush of campaign contributions added to the Democrats’ sense of giddiness, perhaps even to a sense of destiny. In addition, Democratic state parties in the battleground states of Arizona, Michigan, Nevada, Pennsylvania and Wisconsin have more cash on hand than do the Republicans. The Democratic strategy now seems to be as much about the positives of Ms. Harris as about the negatives of Mr. Trump. Mr. Biden’s frailty made it difficult to make that case for him when he was still a candidate.

All this is almost enough for Democrats to start humming “Happy Days Are Here Again,” the Milton Ager and Jack Yellen song that rocketed Franklin Delano Roosevelt to the presidency in 1932.

“This is crazy unity, but it’s the result of the Democrats feeling they are facing an existential threat in Trump and the fact that there is so little time,” said Aram Goudsouzian, a University of Memphis historian. “The danger for Harris and Walz is that they haven’t faced much of a test, and certainly not in primaries. Contested primaries can serve as a way for a party to figure out whether a candidate is strong enough to go on.”

That was the disadvantage vice-president Hubert Humphrey faced in 1968. Having entered the presidential race after Lyndon B. Johnson withdrew on March 31, it was too late for him to join the important contests in New Hampshire, Nebraska, Oregon and California. Mr. Biden arguably was strengthened by outlasting the competition in the 2020 primaries.

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But divisive primaries – a gauntlet Ms. Harris was spared – can also injure a campaign, sometimes mortally. In 1964, the primary challenges that governor Nelson Rockefeller of New York and ambassador Henry Cabot Lodge Jr. (in a remarkable write-in effort) mounted, plus the convention challenge of governor William Scranton of Pennsylvania, hurt the already weakened candidacy of senator Barry Goldwater, who lost to Mr. Johnson in a landslide. The bitter 1980 challenge senator Edward Kennedy of Massachusetts brought to the re-nomination of president Jimmy Carter was a factor in Mr. Carter’s defeat at the hands of Ronald Reagan.

Badly divided parties often have bad electoral outcomes. When former president Theodore Roosevelt bolted from the Republicans in 1912 and mounted an independent campaign under the banner of the Progressive Party, known informally as the Bull Moose Party, the eventual GOP nominee, president William Howard Taft, finished third in an election in which Governor Woodrow Wilson of New Jersey prevailed.

Party division contributed to defeat in 1924, when the Democrats required 103 ballots to select John W. Davis, a former member of the House of Representatives turned diplomat, only to lose the election to president Calvin Coolidge. The Democrats had the same result in 1928, when the party’s standard bearer, New York governor Al Smith, the first Catholic nominee of a major American political party, was unable to resolve divisions over his faith and the conviction of some party members that he was too conservative.

And while a divided party is a losing party, there is no guarantee that a united party is a winning party.

That is the threat the Harris campaign must face. The apparent Democratic unity does not mean that Ms. Harris faces no threats from elements of her party, particularly from Democrats incensed that the U.S. has continued to arm Israel despite rising Palestinian casualties and a deteriorating humanitarian situation in the Gaza Strip.

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Even so, party unity is a vital asset in American politics.

Strong Democratic unity produced important victories in 1932 and 1936 (Franklin Delano Roosevelt) and strong Republican unity produced strong victories in 1952 and 1956 (Dwight Eisenhower). A unified Democratic Party produced unusual enthusiasm for Governor Bill Clinton of Arkansas in his 1992 campaign against president George H.W. Bush, and for senator Barack Obama of Illinois against senator John McCain in 2008 – a campaign that former senator Paul Kirk of Massachusetts, himself a one-time Democratic national chairman, described in an interview as “almost a religious experience.”

There have been some rare occasions when both parties have gone into an election unified.

One was the 1960 election, won by John F. Kennedy. His eventual rival, vice-president Richard Nixon, was able to tamp down a rebellion from the left by Mr. Rockefeller in time to go into the party’s convention in Chicago with a united party behind him.

“A united party means that the nominee doesn’t have to worry during the general election campaign about factions,” said Antoine Yoshinaka, a political scientist at the University at Buffalo. “That’s the situation now for the Democrats right now. But it’s important to remember that the other party left its convention unified, too.”

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