David Shribman is the former executive editor of the Pittsburgh Post Gazette.

The emphasis the two major presidential campaigns are putting on swing states understates the playing field for the 2024 election. It’s the swing voters inside the swing states that matter.

For most of this election year, the vast majority of Americans have been frozen in place, largely depending on their views of former President Donald Trump, but also on their perceptions of President Joe Biden’s ability to perform presidential duties for four more years. But now Biden is gone from the equation, Vice President Kamala Harris is atop the Democratic ticket, and a fresh calculus is taking form.

The new campaign has a new character. It is, to be sure, a very close race. But Harris is surging nationally and in the six swing states. In three of them — Michigan, Wisconsin and Pennsylvania — she has an edge, according to the latest New York Times/Siena College poll. With Biden in the race, Trump had held the lead there. No longer, though most of the figures are within the margin of error.

As recently as last month, about 1 in 4 Americans remained undecided. Now the number is down to about 1 in 7, according to the most recent Wall Street Journal poll. “These are people who need to be reached with information about the candidates and the issues, as they are more likely to be disengaged,” said Tova Wang, director of research projects in democratic practice at Harvard’s Ash Center for Democratic Governance and Innovation. “It is less likely that the political parties will reach out to them, because they’re not on their lists. We are talking about people who are less plugged-in, and the campaigns need to broaden their outreaches.”

These voters — actually, let’s call them potential voters — are the ultimate prize in the ultimate contest. And the Democratic National Convention that opened Monday in Chicago is a fortuitous, and potentially highly valuable, forum for Harris and her running mate, Gov. Tim Walz of Minnesota, to engage some of the unengaged — and to swing some of the swingers.

Because, as Benny Goodman, Bing Crosby, Count Basie and the Andrews Sisters — they’re all dead, but their music lives on — might have put it, employing the title of a popular 2003 musical retrospective, “It’s Swing Time.”

The Glenn Miller selection in that old-timey collection is called, poignantly, “Pennsylvania 6-5000” — swing music with a title that includes the biggest swing state. The Ella Fitzgerald recording captures the Harris-Walz ticket’s soundtrack perfectly: “On the Sunny Side of the Street.” Forget the fact that Pennsylvania 6-5000 was actually a New York telephone number and that the second line of the song (“Leave your worries on the doorstep”) is peculiarly unsuited to our times. It’s Pennsylvania and the sunny, upbeat syncopation of Harris and Walz that are the most important variables in the 2024 campaign.

The return to normality in a decidedly not normal campaign has substantially helped the Democrats.

Ordinarily, party members stick close to their nominees, veering away only occasionally — as they did with two compelling personalities: Ronald Reagan in 1980, when Democrats saw virtue in the former California governor’s optimism, and Barack Obama in 2008, when Republicans found the Illinois senator, and the prospect of the first Black president, appealing.

The problem for the Democrats in 2024 was that party members were resistant to the Biden entreaties, in large measure because of the president’s clear infirmity. That problem is over.

Now, 14 out of 15 members of both parties are prepared to vote for their respective nominees, according to the Journal poll. That, and the cheerful positivity of the Harris-Walz team, are the principal reasons for the Democrats’ surge.

But surges don’t last forever. Sometimes they peter out, sometimes they reach a plateau and glide to the finish line, sometimes outside developments — maybe this time in the Middle East, maybe in next month’s debate, maybe a whopper revelation about J.D. Vance or an alarming disclosure about Walz — interfere. The Wall Street financiers who have flocked to the Democratic ticket would counsel Harris: Even bull markets stutter, and sometimes they turn into bears.

But first the Democrats have an unusual opportunity. None of the 25 major-party political conventions held in Chicago has offered this sort of fortune for a candidate. Not for Abraham Lincoln, who was nominated there in 1860, nor for Franklin Delano Roosevelt in 1932, nor even — if you’re worried about the influence of a third party, in this case Robert F. Kennedy Jr. — for Theodore Roosevelt, who became the Bull Moose Progressive nominee in 1912 in Chicago en route to finishing in second place, leaving the Republican incumbent president, William Howard Taft, in third place.

Party conventions customarily provide what political professionals call a polling “bump,” accounting for an average of about 10 percentage points. Sen. John F. Kerry, the Democrats’ 2004 nominee, didn’t get one, nor did former Vice President Biden in 2020. But Democratic Gov. Bill Clinton got a huge one (about 30 points, though it was inflated by the withdrawal of Independent candidate Ross Perot during the proceedings), and so did Democratic Vice President Al Gore (whose convention erased the 16-point advantage then held by Gov. George W. Bush).

The question is whether the Chicago convention will turbocharge the Harris campaign. That is not inevitable. But, the Trump team must fear, it is possible.

“You can see why the convention [can have] that effect — lots of uninterrupted media exposure, a chance to produce a narrative for what your party can do, lots of celebration of the nominee, compelling visuals and speeches,” said Aram Goudsouzian, a University of Memphis historian.

The Republicans made good use of their convention, crafting a vibrant sense of unity and purpose. The Democrats have that sense of unity and purpose already.

Their task is to build on it.