(Reuters) - More than a dozen times in the past seven days, President Donald Trump has alleged, contrary to evidence, that the recount of Florida’s elections for governor and the U.S. Senate has been marred by fraud.

FILE PHOTO: U.S. President Donald Trump talks to reporters prior to departing for Paris, France from the South Lawn of the White House for in Washington, U.S., November 9, 2018. REUTERS/Kevin Lamarque/File Photo

“Many ballots are missing or forged,” he tweeted on Monday. “Ballots massively infected.”

The unsubstantiated allegations could help Trump bolster a narrative that appeals to his core supporters ahead of his expected 2020 bid for a second term - that of an aggrieved president at risk of being cheated by what he often decries at rallies as an unfair system.

It may also stoke doubts among his loyal base of supporters over the legitimacy of elections - a potentially dangerous tactic, especially if he were to adopt a similar stance in response to razor-close results in the 2020 presidential election.

“For a long time, the American democratic process has been based on the idea that if an election has an outcome that one side doesn’t like, it’s still considered legitimate,” said Tom Pepinsky, a professor of government at Cornell University and senior fellow at the Brookings Institution think tank. “If there’s ever been a time for elected politicians to draw a line in the sand ... this is it.”

Fellow Republicans including Florida’s outgoing governor, Rick Scott, whose Senate campaign is the subject of a recount mandated under state law, incumbent U.S. Senator Marco Rubio and lower-ranking officials have amplified Trump’s charges.

Researchers have long concluded that voter fraud is extraordinarily rare in U.S. elections.

The final result of the match-up between Scott and three-term Democratic incumbent Bill Nelson will not affect the balance of power in the Senate, where Republicans increased their majority in the Nov. 6 elections even as Democrats seized control of the House of Representatives.

“What we’re seeing now is a new and potentially dangerous development,” said Jonathan Brater, a voting rights lawyer at New York University’s Brennan Center for Justice, referring to a call by Trump on Florida officials to abandon the recount.

“Not only would that tactic risk throwing out legitimate votes if it is successful, it also sows seeds of doubt about our entire democratic process in the minds of millions of people.”

Narrow margins of victory for Scott and Republican gubernatorial candidate Ron DeSantis over Democrat Andrew Gillum triggered the recount under state law last week. It stirred memories of Florida’s disputed results in the 2000 presidential election that Republican George W. Bush ultimately won after the U.S. Supreme Court stopped the recount.

Trump has said the Florida election should be called in favor of Scott and DeSantis because “large numbers of new ballots showed up out of nowhere, and many ballots are missing or forged.” He cited no evidence.

A senior White House official said Trump was pressing the issue out of real concern.

“He genuinely thinks there is corruption there,” the official said. Another source close to the White House said the president was following the lead of Scott and Rubio.

“He’s echoing in his Trump way the message on this, which is to be aggressive,” the source said.

‘IT’S RIGGED’

Florida law enforcement officials have said they have seen no evidence of fraud during voting or the recount. Vote totals often change even after an election, as absentee, provisional and overseas ballots are recorded.

Trump supporters who protested on Tuesday outside county offices where votes were being counted rejected that idea.

“It’s rigged, this is absolute system corruption and everybody is playing dumb,” said Sofia Manolesco, a 49-year-old fitness trainer who said she had voted for all Republicans on her ballot and wore a red “Trump 2020” hat.

Following his 2016 victory, Trump falsely claimed that millions of illegal immigrants had voted for Democrat Hillary Clinton.

U.S. politicians, particularly Republicans, have long cited concerns about fraud to justify laws restricting access to polls. But independent researchers have documented only a handful of cases over the years.

Justin Levitt, a professor at the Loyola Law School in Los Angeles, tracked just 31 credible allegations of voter impersonation in U.S. elections from 2000 to 2014. In that time, more than 1 billion ballots were cast, Levitt found.

There have been some documented instances of problems in this year’s Florida voting that could benefit both parties.

In Broward County, where Democrats typically do well, nearly two dozen rejected ballots were counted with a batch of valid ones. The Florida secretary of state’s office said it had received reports that voters in Bay County, a Republican-leaning area hit hard by Hurricane Michael, may have been permitted to vote by email, which is prohibited under state law.

Douglas Heye, a Republican strategist and former top aide to former Republican House Majority Leader Eric Cantor, said he doubted there would be political fallout from Trump’s rhetoric.

“Trump claimed the last election was rigged, until he won,” he said. “We will likely be talking about a million things between now and the 2020 elections. These tweets may be just a distant memory.”