Every four years, a presidential candidate gazes out over a vast crowd and convinces themselves the White House is there for the taking.

In 2016, the general election candidate drawing the biggest and loudest crowds is Donald Trump.

"We got to Oklahoma, we have 25,000 people. We had 21,000 people in Dallas, we had 35,000 people in Mobile, Alabama. We get these massive crowds," Trump said in Florida earlier this month.

"Look, if she had 500 people I would be surprised," he added, poking fun at Hillary Clinton's more intimate events.

During the primaries, Trump's massive crowds did, in fact, translate to votes. But size is not always a barometer of a campaign's destiny.

In fact, extrapolating electoral prospects from the size of rally crowds is often a misleading metric -- for evidence, look no further than the campaigns of Walter Mondale, Michael Dukakis, John Kerry and Mitt Romney. Bernie Sanders thrilled thousands of people in mega-rallies over the past year and a half as well.

Yet every election, candidates and aides, seeking silver linings when beset by bad polls, indulge the wishful thought that bulging rallies will mean a stampede at the ballot box.

Often, they tout that mystical, yet unquantifiable, political commodity: Momentum.

"Momentum's a word from physics that got hijacked by journalists and political operatives to sound scientific," said Sam Wang, a professor of neuroscience who runs Princeton University's Election Consortium. "What it means is -- 'I am excited by where I am today, I am excited by what is happening.'"

That is not stopping Trump however -- just as it did not stop Sanders supporters who saw his crowds of 20,000 and up in the primary race to argue that a tsunami of enthusiasm for the Vermont senator could overcome Clinton.

Trump supporters are putting their faith in boots on the ground at his rallies.

"They are standing in lines for two hours and (in) 90-degree weather to get in," New York Rep. Chris Collins told CNN's Brianna Keilar this week.

"This energy you are seeing ... is why the polls mean nothing. This is a turnout election and the energy is behind Donald Trump."

But history shows, that for all its faults, polling is a better predictor of electoral success in a broad electorate, than crowd size.

Still, it's not impossible that Trump's multitudes is a sign the grassroots uprising will confound conventional wisdom yet again. There is an exception to every rule.

Barack Obama's mega rallies in 2008 did turn out to be an indicator of strength and surprising competitiveness in red states like Missouri -- where he once had a crowd of 100,000 people and Indiana, where he attracted 35,000 to a rally in Indianapolis. He ended up losing Missouri by a whisker but peeled Indiana out of the GOP column.

"In 2008, we had rallies with 50,000, 80,000, 100,000 people. I'm not bragging ... sometimes you hear folks say, 'oh, that rally is big.' I say, I don't know ... We had some pretty big rallies," Obama said at a Democratic fundraiser in Austin, Texas, in March, in what may have been a swipe against Trump.

So, if Trump is drawing massive throngs in blue states in October, it might mean something.

It's not surprising that huge crowds can mislead. After all, running for president is a uniquely grueling, absorbing, emotional experience. And tens of millions of people will vote for the losing candidate in any presidential election, so it isn't like candidates don't have plenty of supporters.

Lines of cars choking roads on the way to an event, or thousands of people packed into an airport hangar at midnight in a remote corner of a swing state, can be intoxicating, replenish an ego and convince a weary candidate they have tapped into something intangible. For campaign workers and reporters, stuck in a bubble, massive rallies can promote erroneous conclusions about the state of a race.

But there are plenty of cautionary tales.

In 1984, Democratic nominee Mondale was trailing President Ronald Reagan, but took refuge in the size of his crowds.

"There's something going on in this country and the pollsters aren't getting it," Mondale said in San Francisco, according to The New York Times.

"Nobody who's been with me for the last few days and has seen these crowds, seen their response, seen their enthusiasm, seen the intensity of their response, and how they respond to these issues ... can help but believe there is something happening in this country."

Mondale was correct. Something was happening. A landslide was brewing that would see him win only his home state, Minnesota, and the District of Columbia.

Four years later, Democratic nominee Dukakis was trailing Vice President George H.W. Bush.

"I smell victory in the air, don't you?" the Democratic nominee told a large crowd in Lexington, Kentucky, on his final weekend on the trail.

His wife, Kitty, looked at steadily growing crowds and sensed an upset. "It's just intuition," she said, according to the Times. "It's something in the crowds. Something Is happening."

Bush won the election easily, finishing more than 300 electoral votes ahead.

In 2004, hopes soared on the Kerry plane that massive crowds just before election day -- 80,000 in Madison, Wisconsin, and 50,000 on Cleveland's waterfront, assured victory. But Kerry lost both Ohio and the White House.

In 2012, GOP nominee Romney was misled by crowds which swelled as election day approached.

"Look at the parking garage!" Romney gushed in footage aired in the documentary "Mitt" as he eyeballed a crowd on election day in Pittsburgh.

"Intellectually, I've felt we're going to win this and have felt that for some time, but emotionally just getting off the plane and seeing those people standing there ... I not only think we're going to win intellectually, I feel it as well," Romney told reporters, hours before losing handily to Obama.

Trump, however, is a man who knows something about making a sale -- and that might be giving him pause.

"I know we are leading Florida by a little bit," he mused in a moment of reflection at his Florida rally.

"I don't know why we are not winning by a lot -- Maybe crowds don't make the difference."