David Jackson, and Kevin Johnson

USA TODAY

WASHINGTON — President Trump vowed an "investigation" Wednesday into alleged voter fraud — despite a lack of evidence behind his claim that 3 million to 5 million people voted illegally in last year's election — though aides spoke more in terms of a "study" of the alleged problem.

"I think we to understand where the problem exists, how deep it goes, and then suggest some remedies to it," White House spokesman Sean Spicer said, "but right now to sort of prejudge the process would sort of get in front of the whole need to have it."

Spicer spoke hours after Trump tweeted that "I will be asking for a major investigation into VOTER FRAUD, including those registered to vote in two states," as well as "those registered to vote who are dead (and many for a long time). Depending on results, we will strengthen up voting procedures!"

Trump has claimed that up to 5 million illegal voters caused him to lose the popular vote to Hillary Clinton in last year's election, and he made the assertion again during a Monday night meeting with lawmakers at the White House.

The Justice Department, which would investigate violations of the federal voting rights laws, declined comment Wednesday on the president’s call for an inquiry and his assertion that up to 5 million illegal immigrants voted in the general election.

Prior to Election Day, the Justice Department’s Civil Rights Division deployed more than 500 people to 28 states to monitor voting.

Monitors were dispatched to such far-flung locations as Alaska’s Kusilvak Census Area and Pawtucket, R.I., to Houston and Phoenix.

In the aftermath of the election, Justice officials announced no major incidents of voter fraud, let alone mass numbers of undocumented immigrants attempting to vote.

Lawmakers and election officials — including Republicans — said there is no evidence of such massive fraud, and that Trump is citing reports on poor record keeping, not dead people voting.

There are issues that need to be looked at, Spicer said: "There's a lot of people that are dead that are on rolls," and people "that are on the (voting) rolls in two different states, sometimes in three different states."

In discussing a possible "study" of the matter — "I don't want to call it a task force yet because it's not there yet" — Spicer did cite "this effort underway that can look at the scope of the problem."

Congressional Democrats say Trump is planning to use the claim to institute new voter qualification rules, making in harder for people to vote in general.

Rep. Elijah Cummings of Maryland, the top Democrat on House Oversight and Government Reform Committee, told MSNBC that Republicans may be seeking "another tool and another reason" to ''deny people the vote."

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Cummings also said he is happy to investigate Trump's claims of voter fraud, saying they are unfounded.

Sen. Bob Casey, D-Pa., said that "if this is the beginning of an effort to suppress legitimate voters from exercising their rights, I will fight like hell against it."

Trump's own lawyers have also called his fraud claims into question. In a legal filing on a proposed recount, Trump's team said, "all available evidence suggests that the 2016 general election was not tainted by fraud or mistake."

Voting rights groups also expressed dismay.

Myrna Pérez, director of the Voting Rights and Elections Project at the Brennan Center for Justice at New York University School of Law, said Trump should focus on pushing programs to register more voters and improve access for them, including updated voting machines and more poll workers.

“Voter fraud and threats of voter fraud have been used throughout this country’s history — and very recently in a numerous amount of circumstances — to justify policies that don’t make our elections more secure and suppress voters," she said.

“It’s an incredible distraction and waste of resources when there are known and substantiated areas of improvement needed in our election administration,’’ she said.

Pérez said Trump has launched “inflammatory, unsubstantiated allegations’’ in the past, including claim that the election is rigged if he loses.

Ohio Secretary of State Jon Husted — a Republican — retweeted Trump and said: "We conducted a review 4 years ago in Ohio & already have a statewide review of 2016 election underway. Easy to vote, hard to cheat."

Senate Democratic leader Chuck Schumer of New York said that Trump "ought to realize he's president. Instead of talking about the election or how many people showed up at the inauguration, he ought to talk about how many new jobs he's creating."

Some Republicans have suggested that Trump stop claiming voter fraud, or at least provide evidence of it. "I think that those who allege that have to come up with some substantiation of the claim," said Sen. John McCain, R-Ariz., on MSNBC.

The Senate's top Republican, Mitch McConnell of Kentucky, said there have been incidents of voter fraud, and people have been prosecuted. "It does occur," McConnell said. "There are always arguments on both sides about how much, how frequent and all the rest."

White House spokesman Sean Spicer said Trump stands by his concerns and cited unknown studies that support Trump's thoughts.

"It's a belief that he's maintained for a while," Spicer said, "a concern that he has about voter fraud."

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Contributing: Deborah Berry