WASHINGTON (AP) — A spokesman said President Donald Trump will sign an executive action to commission an investigation into widespread voter fraud, raising the prospect of a federal government probe into a widely debunked claim and sparking alarm among experts and Democrats.

White House press secretary Sean Spicer said Thursday that Trump would sign the order in the Oval Office but did not provide more details as to what it might entail.

Trump had announced in a pair of tweets early Wednesday that a "major investigation" will look at those registered to vote in more than one state, "those who are illegal and ... even, those registered to vote who are dead (and many for a long time)." Depending on results, the Republican tweeted on his sixth day in office, "we will strengthen up voting procedures!"

He went further later, claiming: "You have people registered in two states. They're registered in a New York and a New Jersey. They vote twice."

"There are millions of votes, in my opinion," Trump told ABC. "Of those votes cast, none of them come to me. None of them come to me."

But House Oversight Committee Chairman Jason Chaffetz on Thursday broke with Trump, saying he sees no evidence of voter fraud in the 2016 election and says his committee won't investigate it.

The Utah Republican says Trump is free to order the Justice Department to investigate the issue, but he's not interested in launching a congressional inquiry. Chaffetz told reporters at the congressional Republican retreat in Philadelphia that voting happens at the local level, adding "I don't see any evidence" of widespread fraud.

All 50 states and the District of Columbia have finalized their election results with no reports of the kind of widespread fraud that Trump alleges.

Spicer would not say whether the investigation would be led by the FBI or some other agency. He said Wednesday that its only goal would be "to understand where the problem exists and how deep it goes" and that it would not be limited to the 2016 election.

Spicer suggested a task force could be commissioned to focus on dead people who remained on voter rolls and people registered in two or more states. And he said it could center on "bigger" states where Trump didn't compete during the campaign, singling out California and New York, two Democratic strongholds.

Trump's tweet alarmed Democrats who already believe that moves to tighten voter ID laws are a means to restrict access to the ballot box. Like the president, Trump's pick for attorney general, Republican Sen. Jeff Sessions of Alabama, who could oversee any federal probe, has shown sympathy toward claims of voting fraud.

There is no evidence of widespread voter fraud in last November's election, in which Trump won the Electoral College, but lost the popular vote by nearly 2.9 million votes to Democrat Hillary Clinton. That popular vote tally has fixated Trump, who worries that it has led to questions about the legitimacy of his victory, aides and associates say.

Speaking to congressional leaders Monday, Trump said 3 to 5 million immigrants in the U.S. illegally voted in the election, denying him a popular vote majority. If that claim were true, it would mark the most significant election fraud in U.S. history — and, ironically, would raise the same questions about Trump's legitimacy that he's trying to avoid. No details have been released about the possible probe.

Senior adviser Kellyanne Conway told NBC's "Today" show Thursday that Trump's call for an investigation is not an "ego issue." ''I think everybody's cherry-picking to call this an ego trip," she said. "Why not have an investigation? What's everybody afraid of?"

Trump's own attorneys dismissed claims of voter fraud in a legal filing responding to Green Party candidate Jill Stein's demand for a recount in Michigan, a state Trump won, late last year. Referring to that outcome, the attorneys wrote: "All available evidence suggests that the 2016 general election was not tainted by fraud or mistake."

Sen. Patrick Leahy, a Vermont Democrat on the Senate Judiciary Committee, said any inquiry should instead be focused on Russian interference in the election, saying: "President Trump's call for an investigation into his latest conspiracy theory is absurd, even for him."

Multiple academic studies, and federal judges, have cast doubt on the prevalence of voter fraud, though an argument that should be based on math has frequently divided along partisan lines.

In one often-cited study, Justin Levitt, an election law expert at Loyola Law School in Los Angeles, identified just 31 instances of potential impersonation out of more than 1 billion votes cast in general, primary, special and municipal elections throughout the country between 2000 and 2014.

Though there are invariably mistakes, clerical errors and instances of ineligible people allowed to register, "there's absolutely no information to indicate that there were problems on the scale that he's alleging," said Levitt, who later was the Justice Department's top elections lawyer under former Attorney General Loretta Lynch.