PHILADELPHIA — America is about to endure the closest, nastiest, most unpredictable presidential election in more than three decades.

Not since Jimmy Carter and Ronald Reagan ran against each other in 1980 has the choice been so stark, the warnings from each candidate about the other so dire, the likely outcome so murky.

As this year’s political conventions end, there is no clear victor. But watch upcoming polls. The leaders in the first polls conducted after Labor Day, which in most recent election cycles is the first after both conventions end, has won the White House every election year since 1952.

“That’s when the dust settles. That is the person who ends up taking the oath of office,” said Lee Miringoff, director of the Marist Institute for Public Opinion, at a breakfast hosted this week by McClatchy.

There is one exception. In 1980, the parties were tied at Labor Day, and the election didn’t swing until the final days.

On paper, Clinton wins if she follows the age-old Democratic playbook: Make sure African-Americans, Latinos, women and labor union members turn out in big numbers. Then she needs to add the liberals and young voters who so adamantly favored rival Bernie Sanders, voters who still need convincing.

“We need now to talk to people one on one,” said Sen. Debbie Stabenow, D-Mich.

Trump wins if he can keep the campaign focused on the anti-establishment narrative that boosted him from long-shot outsider to nominee, a plotline that helped boost him a bit despite a discordant GOP convention last week.

“There’s something different this year,” said Brandon Bell, chairman of the Republican Party in Rhode Island. “People are fed up.” But Trump also needs still-wary mainstream Republicans to back him.

Clinton, her supporters said, has to leave Philadelphia relentlessly reminding Democrats and undecided independents, who make up about 20 percent of the electorate, of her history fighting for their causes — and by painting Trump as unusually dangerous.

“Tell everyone to make a reality check,” said Rep. Al Green, D-Texas, whose congressional district is about one-third African-American. Remind minority voters, he said, of the party’s history of strong support for civil rights.

Tim Kaine, the Democrats’ vice presidential nominee, put the choice in stark terms Friday. “The thing I do best is when the civil rights lawyer in me gets engaged. This is a civil rights battle,” he told a Democratic National Committee meeting.

Getting African-Americans to turn out in the sort of numbers President Barack Obama got, though, is going to be tough.

“There is only one Obama,” said state Rep. Gilda Cobb-Hunter of Orangeburg, S.C., though he can be a big help if he gives Clinton the sort of full-throated support he offered at the convention.

Obama got 93 percent of the black vote in 2012 while winning 41 percent of the white vote. Black turnout was 66.2 percent, 2 points higher than whites. In 2004, black turnout was 60 percent. Clinton this year has outpolled Trump among African-Americans by about 7 or 8 to 1.

Trump’s path to victory has two lanes: Pound away at the anti-establishment message, and woo back Republicans who have been sharply critical and stayed away from last week’s convention.

That remains difficult. Gov. John Kasich of Ohio, who didn’t attend the convention in his home state, still hasn’t endorsed Trump, or even spoken well of him. He recently told The Associated Press that Trump “would have to change everything he says” before that could happen.

More importantly, Trump needs to keep the campaign narrative focused on the throw-the-bums-out mood that rocketed him from politically nowhere to the GOP nominee.

Trump’s other challenge is to keep people outraged for three more months. Circumstances can help. WikiLeaks is promising more data releases aimed at embarrassing Clinton.

Democrats had better be ready, said Gov. Terry McAuliffe of Virginia, a former Democratic chairman. “I hope they’re going through every single email that they’ve had and to look at it and get on top if it ahead of time,” he said this week in Philadelphia.

Democratic leaders were split Friday on the controversy’s impact. “I’m hoping we’ll learn from this and be more transparent,” said Jeri Shepherd, a DNC member from Colorado. Asked whether it would have an impact on elections, she said, “Time will tell.”

But Maureen McKenna, a DNC member from Sebring, Fla., was more upbeat.

“The players may change,” she said, “but the system remains the same.”

There’s potential for another sort of email drama. Republicans won’t let voters forget about Clinton’s use of a private email server while secretary of state.

“We’ve known from the beginning of this campaign that Clinton’s personal political history was going to be a drag on her candidacy,” said Patrick Murray, director of the Monmouth University Polling Institute in New Jersey.

Clinton will counter by painting Trump as inept, incompetent and all but insane.

“Imagine him in the Oval Office facing a real crisis. A man you can bait with a tweet is not a man we can trust with nuclear weapons,” she said in her convention speech Thursday. “I can’t put it any better than Jackie Kennedy did after the Cuban Missile Crisis. She said that what worried President Kennedy during that very dangerous time was that a war might be started — not by big men with self-control and restraint, but by little men — the ones moved by fear and pride.”

Stoking fear about an opponent is a time-tested tactic. President Lyndon Johnson’s 1964 “daisy ad” suggested Republican Barry Goldwater would involve the nation in a nuclear war. In 1980, Jimmy Carter made the same insinuations. In his October, 1980, debate with Ronald Reagan, Carter used the word “dangerous” six times to describe his Republican opponent.

The biggest bloc of persuadable voters this year is those under 30. Clinton needs them more, since they historically have leaned Democratic, but a Harvard Institute of Politics poll this month showed Clinton viewed unfavorably by 60 percent of those aged 18 to 29.

Nearly one-fourth said they may not vote, and polls suggest one-fourth of all young voters are considering Libertarian Gary Johnson or the Green Party’s Jill Stein.

John Della Volpe, the institute’s polling director, conducted two town hall meetings with young voters this month. One was in Cleveland, site of the Republican convention, and one was in Philadelphia, where Democrats met.

Regardless of party, “They’re basically saying, ‘Please, please motivate me,’” he found.

That’s how Dustin Chiang, a sophomore college student from Fremont, Calif., felt. “Over the course of the convention I’ve become more for her,” he said.

Chiang, who helped organize the Philadelphia town hall, estimated that about one-third of the participants were “all in for Clinton,” one-third were reluctant and some said they’d never vote for her.

“That really showed the divisions out there,” he said.

He estimated Clinton needs to win about 60 percent of that vote — which will be a challenge since Democratic rival Bernie Sanders won overwhelmingly in primaries among younger voters — — while Trump needs about 45 percent. Obama won 66 percent in 2008 and 60 percent four years ago.

Trump’s swashbuckling style gets him a serious look, despite leading a party whose views on gay marriage and other social issues are not aligned with those of most young voters.

Trump is regarded as sympathetic to gay rights. “His appeal doesn’t involve the social issues,” explained Jeremy Wiggins, 21, a Trump delegate from Missouri. To voters such as Wiggins, he’s fresh, and most important, eager to shake up a system these voters often see as blind to their needs.

The next pivotal campaign moment is likely Sept. 26, when Clinton and Trump are scheduled to debate in Hempstead, N.Y. Two more debates are to follow, in St. Louis on Oct. 9 and Las Vegas Oct. 19. Vice presidential candidate Mike Pence, a Republican, and Tim Kaine, a Democrat, are to debate Oct. 4 in Farmville, Va.

Chances are Trump and Clinton are too well-known, and too widely disliked, to suddenly become likeable figures in the next few months. The debates are their single best chance to soften those images.

Odds are they won’t, and as a result, said Miringoff, “there’s an awful lot of negative voting going on.”

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POLLS TO WATCH

Gallup’s first Labor Day polls since 1952 show that except for 1980, whoever led in the first post-Labor Day poll won the presidential election.

Here are the winners, and their leads in that poll:

Dwight Eisenhower, 1952, 15 points.

Eisenhower, 1956, 11 points.

John F. Kennedy, 1960, 1 point

Lyndon Johnson, 1964, 36 points.

Richard Nixon, 1968, 12 points.

Nixon, 1972, 28 points.

Jimmy Carter, 1976, 14 points.

Carter and Ronald Reagan, 1980, tied.

Reagan, 1984, 19 points.

George H.W. Bush, 1988, 8 points.

Bill Clinton, 1992, 15 points.

Clinton, 1996, 17 points.

George W. Bush, 2000, 1 point.

Bush, 2004, 7 points.

Barack Obama, 2008, 6 points.

Obama, 2012, 8 points.