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 Republican 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 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 Republican Party’s 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.”

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.

Chances are Trump and Clinton are too well-known, and too widely disliked, to suddenly become likable 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.”