HEMPSTEAD, N.Y. — The hype surrounding the opening debate here between Hillary Clinton and Donald Trump has approached Super Bowl-size dimensions, with countdown clocks on cable shows and predictions that the TV audience will rival the football championship, even without a halftime show.

But does the opening debate really make that much difference?

If history is a judge, the answer is yes.

That's not the say the candidate judged the unofficial "winner" of the first debate always prevails on Election Day. In the 11 campaigns where there have been televised debates, three contenders who "lost" the opening encounter ended up being inaugurated president the next January. Jimmy Carter in 1976, Ronald Reagan in 1984 and Barack Obama in 2012 did that with comeback performances in the second debate.

But in nine of those 11 campaigns, the first debate did shape the trajectory of the campaign's final weeks in fundamental ways. Sometimes it provided a critical boost for a candidate (John Kennedy in 1960, Reagan in 1980). Sometimes it wounded a contender, leaving a bad impression that just never got fully fixed (Michael Dukakis in 1988, Al Gore in 2000).

The first debate typically has had the most impact when a candidate comes in with something to prove or when the contest is close and fluid.

That is, in campaigns like this one.

Indeed, no presidential election in more than three decades — since Reagan and Carter held their only debate a week before Election Day in 1980 — has a debate had such outsize potential to affect the election.

Republican Donald Trump arrives on stage with something to prove: That he can be presidential as well as provocative. Democrat Hillary Clinton needs to reassure Americans that she has a vision for a nation most of them say is going in the wrong direction. Both contenders face skepticism that they should be trusted with the most powerful office in the world.

And their race suddenly seems as close as a tick.

The modest lead Clinton held a month ago has evaporated. A national Quinnipiac University poll released Monday showed her ahead of Trump by a single percentage point, while a new Bloomberg poll had Trump up two points. Battleground states that Democrats have been counting on to block the path to a Trump victory in the Electoral College also tightened in polls released by CNN/ORC Monday: Clinton was up just one point in Pennsylvania; Trump up one point in Colorado.

Presidential Outlook from USA Today.

"The first debate is the moment many swing voters tune into the race for the first time, and if one candidate clearly prevails, the race can move to them and it may stay there," says Democratic strategist Tad Devine, a top aide to Gore in 2000 and Vermont Sen. Bernie Sanders in this year's primaries. Especially when there's no president running for re-election, "the first debate can be the true turning point from which, for the loser, there may be no return. It's very high stakes."

Strategists in both parties compare Monday's debate at Hofstra University to the pivotal 1980 forum, though they disagree on which candidate should be cast in the role of Reagan, who overcame many voters' reservations with a reassuring demeanor and a compelling closing statement. His devastating and now-iconic question: "Are you better off than you were four years ago?"

"Just like with Reagan, I believe that once Donald Trump is accepted by the American people as somebody who can be president, I think the race is over," says Paul Manafort, a campaign aide to Reagan who was Trump's top strategist at the Republican National Convention. "The convention is the beginning of that."

Some Trump partisans believe the debate will help finish it, as it did for Reagan.

Democratic pollster Peter Hart also sees a parallel with 1980 when many voters didn't want to vote for Carter but weren't sure about Reagan. "It was not until the single debate with President Carter that voters were convinced that Reagan could be a safe choice," he wrote after holding a focus group with a dozen swing voters in the Milwaukee suburbs last month for the Annenberg Public Policy Center at the University of Pennsylvania. "This year, a lot of voters know they do not want Donald Trump as president, but they need to know that they can 'live with' Hillary Clinton for the next four years."

First debates are powerful because it's a time when some voters begin to pay close attention. It is often the first time the candidates stand side-by-side. At an hour-and-a-half, the conversation is more extended and more grueling than any 30-second hour or 10-minute interview.

Ninety minutes is a long time. So is four years.