Trump, without evidence, has said that three to five million people voted illegally for Hillary Clinton in the 2016 presidential election -- similar to the margin of Clinton's popular vote victory over Trump.
"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," he tweeted in late November.
But so far, no evidence has surfaced to justify his claim.
"This is a transparent attempt to justify President Trump's false claim that millions of people voted illegally in the 2016 presidential election. There's simply no evidence of widespread voter fraud in this country. Period," said Sen. Dianne Feinstein, the top Democrat on the Senate Judiciary Committee.
Rep. Elijah Cummings, the top Democrat on the House Oversight and Government Reform Committee, called the voter fraud investigation "a partisan effort" to "squander" taxpayers dollars.
"Today, President Trump is resuscitating his voter fraud investigation in a desperate attempt to retroactively prove his completely baseless claim that up to five million unauthorized immigrants cast illegal votes in the 2016 election," he wrote.
The ACLU, meanwhile, filed a Freedom of Information Act request Thursday
"seeking information that the Trump administration is using as the basis for its voter fraud claims."
The commission will be spearheaded by Vice President Mike Pence and controversial Kansas Secretary of State Kris Kobach. Kobach, who helped on the Trump transition team, is a lightning rod for critics who have accused him of extreme racism and having ties to white nationalists.
Kobach is almost single-handedly responsible for some of the nation's strictest immigration laws in at least a half-dozen states -- he not only writes the laws, but advocates for them and battles on their behalf in court. He is often cited as the chief architect of what Arizona's SB 1070, which was passed in 2010 and was criticized as encouraging the profiling of Latinos and other minorities.
The Arizona law requires police to determine a person's immigration status when there is "reasonable suspicion" that they are not legally in the US; it was partially upheld by the Supreme Court, but had other sections struck down by the court in 2011.
But the effort will be bipartisan. During the daily White House briefing, deputy press secretary Sarah Huckabee Sanders announced additional members -- including Maine Secretary of State Matt Dunlap -- who will be added to a list of about a dozen: Connie Lawson, Indiana's secretary of state; Bill Gardner, New Hampshire's secretary of state; Ken Blackwell, former secretary of state of Ohio; and Christy McCormick, who already serves on the US Election Assistance Commission.
And at least one member says he suspects the President's claims will remain unfounded.
Dunlap, a Democrat, told CNN Thursday he was open to joining the President's commission after another member asked him to. Though he's not sure of a timeline of what the group is doing and when, he suspects that the commission will not find much in the way of voter fraud.
Dunlap said he received no complaints of voter fraud in Maine related to the 2016 election, and he does not have any concerns about voter fraud as nationwide issue.
"I suspect they will remain unsubstantiated claims," he said referring to Trump.
"I'm as far away politically as you can get to the positions of the President and Vice President, but they're elected and in office. Give them the benefit of the doubt," he said on whether he thinks the commission will produce evidence of voter fraud.
And Dunlap is fine with the outcome either way, saying it remains to be seen if the commission is a waste of taxpayer dollars, but his hope is that the commission finds a definitive answer either way.
"I hope it doesn't form a political agenda," he added.
Dunlap isn't concerned that his acceptance to join the commission lends it credibility. In fact, he sees it as a "bully pulpit" to speak about other election concerns he has, specifically the Department of Homeland Security's involvement in the voting process and voter ID laws.
Trump was initially set to sign an executive order looking into the alleged voter fraud on January 26, a few days after taking office. Reporters and photographers were summoned to the West Wing to document the signing, but waited for about 30 minutes amid confusion from aides. At the time, Spicer said the President "got jammed up on some meetings that needed to occur" and that the order would be signed in the next day or two.
In February, one aide said the President was waiting until then-Alabama Sen. Jeff Sessions was confirmed as attorney general before launching an investigation.
The President was asked about the claims again on March 22 during an interview with Time magazine about truth and falsehoods. He was asked about this disputed claim, which he said he would "be proved right about that, too."
"We'll see after the committee. I have people say it was more than that. We will see after we have. But there will be, we are forming a committee. And we are going to do a study on it, a very serious problem," Trump told the magazine.
Senate Majority Leader Mitch McConnell said in February that no federal money
should be spent investigating voter fraud.
"There's no evidence that it occurred in such a significant number that would have changed the presidential election, and I don't think we ought to spend any federal money investigating that," the Kentucky Republican told CNN's Jake Tapper on "State of the Union."
After Trump reportedly told several senators in a February private White House meeting much of the fraud took place in New Hampshire,
White House aide Stephen Miller repeated the claim on television.
But many people in New Hampshire responded, voicing confusion and anger.
Former New Hampshire Republican Party chairman Fergus Cullen took to Twitter to offer a $1,000 reward for evidence of a single illegal vote in New Hampshire by someone coming from Massachusetts. He said no one had offered evidence, and in an interview with CNN's Brooke Baldwin, he blasted the White House's claim.
"The idea that people are coming to New Hampshire to commit fraud on a massive scale like this is just preposterous and it needs to be called out as untrue," Cullen said.
The New Hampshire secretary of state's office said Thursday while there is no evidence of widespread voter fraud, isolated cases do occur.