MINNEAPOLIS - Donald Trump's refusal to say if he'll accept the election outcome was the second most tweeted moment of Wednesday night's debate.

Thursday, he spoke about those comments saying he would accept the outcome of the presidential election if he won.

"It was shocking not only to Democrats who couldn't believe their luck in having another faux pas by the Republican nominee, but it was also shocking to republicans," said Larry Jacobs, a University of Minnesota professor of political science.

He says there's no law requiring presidential candidates to concede and that Trump's options would be limited unless the race is close.

"There are states that have laws about contested elections," said Jacobs, but state laws also vary.

What took so many off-guard is Trump questioning the generations-long tradition we assume will happen, the candidate who loses will accept defeat.

Take John McCain's loss to President Obama in 2008 after an historic and bitter campaign cycle. McCain still called for unity. Even tried to hush people protesting in the crowd that night of his concession speech.

"If Donald Trump does not accept the legitimacy of the outcome the ridicule will fall on him," said Jacobs.

The last time a presidential election was contested was in 2000 with the Florida recount and that went all the way to the U.S. Supreme Court.

Jacobs doesn't think this election will be close enough to trigger a recount if the current poll numbers hold true on Election Day.