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Donald Trump has repeatedly criticized the presidential election as rigged and refused to commit to conceding if he loses the race.

As Election Day begins, some are asking what would happen if Trump loses and declines to concede.

The answer is: Nothing.

There is no legal or constitutional requirement that a losing candidate publicly concede, experts told NBC News.

A refusal to concede in the face of a clear loss would certainly be unusual — and probably controversial — but it would not impact the formal results in any way.

“The act of conceding is not a legal act,” political scientist Rick Hasen said.

“It doesn't have any legal effect as to whether or not someone is declared a winner,” added Hasen, who runs the Election Law Blog.

Related: Could a Contested Election Lead to a Recession?

“There is no legal requirement for the loser to concede,” says Barbara Perry, director of presidential studies at the University of Virginia’s Miller Center.

The entire “concept of a concession speech as we know it,” Perry added, “didn’t exist at the founding” of the nation.

The tradition of concession speeches only became prominent recently as TV addresses became common, which historian John Vile recounts in his book on the practice, “Presidential Winners and Losers: Words of Victory and Concession.”

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“The concession speech itself has no legal status,” said Edward Foley, who runs the election law program at Ohio State University.

“It’s more part of this cultural expectation,” he told NBC News, adding that Trump has clearly “suggested he wouldn’t give the typical concession speech.”

Scott Farris, a historian who studies losing presidential candidates, said Trump is also unusual because he is unlikely to be constrained by his future standing within the party.

“Because Donald Trump is not conventional politician looking ahead to run again, he’s going to have less inhibitions in criticizing the result and the process," he said.

That does not mean, however, that the entire GOP would back Trump’s refusal to concede if the results showed a clear loss.

Experts consulted by NBC News differed on how a non-concession might play out.

Polls show about 27 percent of Trump voters would not accept the results if their candidate lost, compared to 11 percent of Clinton voters, and some analysts believe a refusal to concede makes it far harder for the public to accept the election’s outcome.

“How are we going to solve problems through policies if we have 40-45 percent of the people not accepting the result?” asked Perry, the historian. “Our system could be doomed, and our country could doomed.”

Other experts predicted that given a clear loss, other Republican leaders would concede regardless of what Trump does.

James McCann, a Purdue political scientist who studies concession speeches, said: "Say Hillary Clinton wins and is recognized. If Donald Trump decides not to concede, and he wants to continue charging that the system is rigged, I'd imagine that Mike Pence would concede on the ticket’s behalf."

Other historians say no matter what happens Tuesday night, critical damage was already done when Trump refused to commit to the principle of accepting a clear loss, when pressed by Fox’s Chris Wallace at the third presidential debate.

“To say that he would ‘leave us in suspense,' is really a breach of we normally have in American presidential politics," presidential historian Robert Dallek said.

On the other hand, Trump’s aides tried to calibrate the GOP candidate's unusual stance at the debate by suggesting he was simply reserving the right to challenge close results.

Related: What to Watch on Election Night

All candidates retain that right — regardless of their public statements — and experts consulted by NBC News said its a routine prerogative of both candidates.

“If the election is very close it is perfectly reasonable for a candidate not to concede immediately," University of Florida political scientist Michael McDonald said.

McDonald, who advises The Associated Press on its Election Night calls, explained candidates in very close may sue for recounts or other challenges in the designated certification period, when “election officials check the results.”

If Trump and Clinton are separate by one state, or just a few, then the trailing candidate could refuse to concede and sue over the results.

Unlike a stand-alone refusal to concede, litigation can impact the outcome, as Americans remember from the 2000 election. A refusal to concede combined with legitimate lawsuits, in other words, can impact the outcome.

But it’s a very narrow path.

According to a review of state election laws by NBC News, the largest battleground states require extremely close margins to trigger automatic recounts. (The margin is under one half of a percentage point in Florida, Pennsylvania, Ohio and North Carolina.)

Historically, the results of a race only shift through a recounts when the margin is far less than half a point.

That means both the math and the law make post-election lawsuits an unlikely path to victory — but one that can’t be ruled out until all the results are in.