President Barack Obama says Republican candidate Donald Trump's insistence that the November 8 election is rigged against him is "no laughing matter."
Campaigning for Democrat Hillary Clinton in Miami on Thursday, Obama said Trump's shifting stance on whether he will accept American voters' verdict on the presidential race next month was "dangerous" for democracy.
Trump's latest position, which he enunciated Thursday, apparently in jest to fervent supporters, was to say he absolutely would accept the outcome of the election, as long as it showed he was the winner.
WATCH: Trump on election results at Ohio rally
During the final presidential candidates' debate Wednesday night in Las Vegas, Trump declined to pledge he would accept the election results, instead saying he wanted to keep the country "in suspense."
The candidate's statement conflicted with promises made just hours earlier by his vice presidential running mate, senior officials of his campaign, and his daughter and close adviser, Ivanka Trump.
WATCH: Obama speaks out on Trump
"When you try to sow the seeds of doubt in people's minds about the legitimacy of our elections, that undermines our democracy," the president said in Florida. "Then you're doing the work of our adversaries for them."
Obama tore into Trump for claiming he knew there would be widespread voter fraud and that the election would be rigged, without presenting any evidence — essentially hinting he might not concede the defeat that almost all national public opinion polls predict is awaiting him.
The president also belittled Florida Senator Marco Rubio, who competed against Trump for the Republican presidential nomination.
Rubio was denounced repeatedly by Trump in humiliating terms before he dropped out of the race, yet he later supported Trump and has continued to do so despite the controversy this month over Trump's lewd comments about women and allegations by more than a dozen women that they were victims of Trump's misconduct.
Rubio became better known during the campaign as a target for Trump's scorn than for his platform, with Trump calling the younger man "Little Marco."
"Come on, man," Obama said in a remark aimed at Rubio, who was not present, chiding the senator for declining to turn away from Trump.
WATCH: Trump tells Wallace he'll decide after election whether to accept results
The Republican standard-brearer, a first-time candidate who previously was better known as a real estate magnate and television personality, stunned the nation and left moderator Chris Wallace speechless when he refused to say during Wednesday night's debate whether he would accept the November 8 election results.
"I will look at it at the time," Trump said. When Wallace pressed the issue, asking whether he would accept a Clinton win, Trump replied, "I will keep you in suspense."
Campaigning Thursday in Ohio, Trump said he would "totally accept the results of this great and historic presidential election, if I win."
Later, he modified his position slightly, saying: "I will accept a clear election result, but I would also reserve my right to contest and file a legal challenge in the case of a questionable result."
A loser of a presidential election is expected to graciously concede defeat and congratulate his opponent — an American political tradition begun more than 200 years ago — even when vote results are extremely close.
WATCH: Biden on Trump's 'rigged' election comments
In 2000, perhaps the closest U.S. presidential vote ever, it took 34 days, many recounted ballots and a legal battle that extended to the nation's highest court before it was decided that Republican George W. Bush had defeated Democrat Al Gore, who was former President Bill Clinton's vice president.
Despite weeks of exceptional political tension, Gore conceded defeat to Bush cordially and told his campaign workers and supporters to accept the results of the election.
Election fraud has been extremely rare in U.S. politics in modern times. A study by Deputy Assistant Attorney General Justin Levitt of voting between 2000 and 2014, a period when more than a billion ballots were cast by American voters, shows there were just 31 individual cases of voter fraud — of someone trying to impersonate an absent voter in order to cast a fraudulent ballot.