US President Donald Trump at the Inaugural Law Enforcement Officers and First Responders Reception in the Blue Room of the White House in Washington on Sunday. Reuters/Joshua Roberts President Donald Trump tweeted Wednesday that he would ask for a "MAJOR INVESTIGATION into voter fraud." He has faced days of criticism over his repeated false claims that millions of illegal ballots were cast in the presidential election.

"I will be asking for a major investigation into VOTER FRAUD, including those registered to vote in two states, those who are illegal and even, those registered to vote who are dead (and many for a long time)," Trump said. "Depending on results, we will strengthen up voting procedures!"

There is no evidence to support Trump's repeated assertion that he lost the popular vote to Democrat Hillary Clinton in November's election because people voted illegally, independent experts and analysts have said.

Ohio Secretary of State Jon Husted, a Republican official charged with overseeing state voting procedures, said Wednesday that Ohio "conducted a review four years ago ... and already have a statewide review of 2016 election underway."

"Easy to vote, hard to cheat," he continued.

Trump tweeted the comments one day after his press secretary, Sean Spicer, defended the president's baseless claims of widespread voter fraud. Trump secured the presidency with 306 electoral votes to Clinton's 232, but Clinton won the popular vote by nearly 3 million.

The assertions of voter fraud, which Trump first made shortly after the election, gained traction this week after Trump, as president, repeated them during a closed-door meeting with congressional leaders on Monday.

"The president believes what he believes," Spicer told reporters on Tuesday when asked what evidence Trump had that millions of illegal ballots were cast. "I think he's stated his concerns of voter fraud and people voting illegally during the campaign. He continues to maintain that belief based on studies and evidence that people have presented to him."

Reporters asked Spicer why Trump wouldn't launch an investigation into voter fraud if he believed it had affected the election, to which Spicer replied, "Maybe we will." Spicer said the president believed that as many as 3 million to 5 million people voted illegally in November.

'Zero evidence of fraud'

Trump and his aides have cited a 2012 Pew study and a debunked report published in The Washington Post in 2014 to back up the false claims that voter fraud affected the election.

During a rally in October, Trump cited "the highly respected Pew," which he said found that "there are 24 million voter registrations in the United States that are either invalid or significantly inaccurate."

"There are 1.8 million dead people that are registered right now to vote," Trump said. "And folks, folks, some of them vote."

But the author of that study, David Becker, has repeatedly pushed back against Trump's claims that there was widespread voter fraud in the 2016 election.

"As I've noted before, voting integrity better in this election than ever before. Zero evidence of fraud," Becker tweeted on Tuesday.

The president tweeted in late November: "In addition to winning the Electoral College in a landslide, I won the popular vote if you deduct the millions of people who voted illegally. Serious voter fraud in Virginia, New Hampshire and California — so why isn't the media reporting on this? Serious bias - big problem!"

Becker, the author of the Pew study, replied to Trump on Twitter: "We found millions of out of date registration records due to people moving or dying, but found no evidence that voter fraud resulted."

'There has been a tendency to misread our results'

Trump's transition spokesman, Jason Miller, subsequently pointed to a disputed report published by a Washington Post blog in 2014 that said 14% of noncitizens said they were registered to vote in 2008 and 2010.

Trump's son Eric also cited the Washington Post report in an October interview with ABC after he was asked whether his father would accept the results of the election if he lost.

"My father will accept it 100% if it's fair ... 14% of all noncitizens in this country are registered to vote," Trump claimed.

The authors of that study have downplayed its findings, saying that while "we stand by our finding that some non-citizens have voted in recent elections," the study's findings about noncitizen voting had been exaggerated and misrepresented.

One of the study's authors, Old Dominion University professor Jesse Richman, wrote a response to the use of his data by the Trump campaign, saying that "on the right there has been a tendency to misread our results as proof of massive voter fraud."

The Washington Post blog was ultimately revised with an editor's note: "The post occasioned three rebuttals (here, here and here) as well as a response from the authors. Subsequently, another peer-reviewed article argued that the findings reported in this post (and affiliated article) were biased and that the authors' data do not provide evidence of non-citizen voting in U.S. elections."