Donald Trump’s top campaign aides say he still has time to turn things around after his sustained deficit in national and swing-state polling.

If history is any precedent, though, it might be tough for him: no candidate in recent history has faced the kind of polling deficit Trump has at this point in the race and gone on to win the election in November.

Thinking about the course of a campaign, that makes sense: by Labor Day, with just 70 days to go until Election Day, many of the biggest moments for a candidate are already over. Trump and Clinton declared their campaigns more than a year ago, and both have now held their party conventions.

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There’s always the potential for unexpected events—like the 2008 financial crash, or Hurricane Sandy in October 2012—but the only other major scheduled moments left for the candidates this year are the presidential and vice presidential debates in September and October.

That said, political observers caution that in a year that has already been unprecedented in so many ways: Trump’s candidacy has reshaped the Republican party and challenged ideas of modern campaigning on the national scale, so it wouldn’t be entirely surprising if he also shakes up the conventional wisdom about polling.

Anthony Salvanto, director of elections for CBS News, said past polling data is “instructive but not predictive.”

“I would say no one should think that anything in August is permanent,” said Anthony Salvanto, director of elections at CBS News. “Historical comparisons aren’t always useful because every year is different, and we have more good polling now than we ever have before. Who knows what things might have shown 20 or 30 years ago when there just wasn’t as much of it?”

With that in mind, here’s a look at where things stood at this point in previous presidential races:

POLLING TODAY

Hillary Clinton has led in virtually every major public poll since mid-July, a trend that has solidified in the weeks since both major party conventions in Cleveland and Philadelphia.

A Monmouth University poll released Monday found Clinton with a 7-point lead, 49 percent to 42 percent; one from Quinnipiac released late last week had Clinton leading by 10 points, 51 percent to 41 percent. Others in the last few weeks have put her lead at 8 points (NBC), 6 points (Bloomberg) and 8 points (ABC/Washington Post). Even when Libertarian candidate Gary Johnson and Green Party candidate Jill Stein are included, Clinton still leads by mid-single digit margins.

“The swing state polling is pointing toward Clinton at this point,” Salvanto said. “Trump’s task now is to not just tilt a close state or two—he has to actively reverse Clinton’s leads.”

To look back just at the last few elections, it’s true that the candidate leading in most August polling was also the candidate who ended up winning the popular vote in November.

2012

Four years ago, in August of 2012, national polls showed a close race between President Obama and Mitt Romney, but one that tended to favor Mr. Obama by several percentage points. A CBS News poll conducted in late August that year found Mr. Obama up by 1 point, 46 percent to 45 percent; polls from ABC/Washington Post and AP/GfK also released polls finding Mr. Obama with a 1-point lead.

Some polls also found Mr. Obama with a 2-point lead (CNN/ORC) and a 4-point lead (NBC/Wall Street Journal). Fox News’ August poll had Romney leading by 1 (45 percent to 44 percent), but it was one of the few polls that month to give Romney the advantage.

Mr. Obama went on to win the national popular vote by 4 percentage points, 51 percent to 47 percent.

2008

In 2008, Mr. Obama’s lead over Republican Sen. John McCain in August was small but pronounced. Many polls gave him a 3-point lead, including CBS News/New York Times (45 percent to 42 percent), NBC/Wall Street Journal (45 percent to 42 percent), Fox News (42 percent to 39 percent) and USA Today/Gallup (48 percent to 45 percent. Others showed Mr. Obama up by 4 points (ABC/Washington Post) or 5 points (Quinnipiac); just one, from CNN, found the two candidates tied at 47 percent apiece.

A CBS News poll conducted late in August gave Mr. Obama one of his biggest leads of the year: he was ahead 8 points, 48 percent to 40 percent.

Mr. Obama won the election that November by 7 points, 53 percent to 46 percent.

2004

Four years before that, in 2004, the polling picture in late August was a bit more mixed: then-President George W. Bush still led in more polls than Massachusetts Sen. John Kerry did, but the race looked to be a margin-of-error contest in many polls.

Several polls found Bush with a 2-point lead, including surveys from Time Magazine, CNN/USA Today/Gallup and NBC/Wall Street Journal. A few, including ABC/Washington Post, found Bush and Kerry tied. A CBS News poll in mid-August of 2004 put Kerry ahead by one point, 46 percent to 45 percent.

That year, Mr. Bush solidified his lead in late August and early September, which was when the Republican convention was held in New York City. He defeated Kerry by about 3 percentage points that November, 51 percent to 48 percent.

2000

In 2000, then-Vice President Al Gore had a small lead over Texas Gov. George W. Bush in most national polling—and while he didn’t go on to win the election, he did win the popular vote against Bush. A CBS News poll released that month had Gore up by 1 point (45 percent to 44 percent); one from NBC had him leading by 3 points (46 percent to 43 percent); one from ABC/Washington Post had Gore up 4 points (48 percent to 44 percent).

Ultimately, Gore edged out Bush in the popular vote, 48.4 percent to 47.9 percent; Bush won the election with 271 electoral votes to Gore’s 266.