U.S. President Donald Trump is calling for a "major" investigation into voter fraud in November's presidential election, reinforcing his belief that voting by millions of undocumented immigrants cost him a victory in the national popular vote.
Trump announced plans for the probe Wednesday morning on Twitter.
Press secretary pressed on issue
But in a Wednesday briefing, White House spokesman Sean Spicer expanded the scope of the upcoming investigation. "This is not just about the 2016 election, this is about the integrity of our voting system."
Spicer did not offer specifics about who would conduct an investigation or when it would begin. But he said investigators would explore irregularities such as outdated voter registration rolls and voters who have relocated and are registered in more than one location.
WATCH: Spicer responds to reporter question on voter fraud
Spicer said more details will be available "as the week goes on."
The issue began to garner more attention Tuesday, when Spicer reiterated to reporters Trump's claim three to five million immigrants illegally voted for his opponent, Democrat Hillary Clinton.
Election officials who have analyzed the November 8 vote say there were almost no indications of voter fraud, certainly not on the scale Trump cites.
"We are not aware of any evidence that supports the voter fraud claims made by President Trump, but we are open to learning more about the Administration’s concerns," the National Association of Secretaries of State (NASS), which represents 40 U.S. state election officials, said in a statement Tuesday. "In the lead up to the November 2016 election, secretaries of state expressed their confidence in the systemic integrity of our election process as a bipartisan group, and they stand behind that statement today,” NASS added.
Trump lawyers have stated in writing that evidence of fraud was nonexistent in the November 8 election. In a filing last year against Green Party presidential candidate Jill Stein's attempts to force a recount of votes in Michigan, they concluded, "All evidence suggests the 2016 general election was not tainted by fraud or mistake."
Ryan: no evidence of rampant voter fraud
Republican House Speaker Paul Ryan told reporters he has seen no evidence of rampant voter fraud in the 2016 election. Republican Senate Majority Leader Mitch McConnell told reporters voter fraud does happen, adding, “There are always arguments on both sides about how much, how frequent and all the rest."
South Carolina Senator Lindsey Graham, one of the Republican presidential contenders Trump defeated in the run-up to the national election against Clinton, called on the president to stop repeating the claim, saying that if he has evidence of fraud, "he needs to disclose why he believes that."
Clinton won the popular vote count over Trump by nearly three million votes. But he won where the Electoral College, the system the United States uses to pick its presidents, with the state-by-state election results determining the winner, not the national vote total.
Sanders: fraud claims 'nonsensical', 'delusional'
Another presidential candidate, Independent Senator Bernie Sanders, who ran as a Democrat against Clinton, told reporters Trump’s voter fraud claims are “nonsensical” and "delusional." Sanders expressed fear that Trump is laying the foundation for more restrictive voting laws.
"What I fear about that statement, and that is something we should all worry about ... he is sending a message to every Republican governor in this country to go forward with voter suppression." Sanders called voter suppression the greatest democratic crisis facing the United States.
The investigation into voter fraud could be led by the Justice Department, which enforces voting rights laws. But the nation's top law enforcement agency analyzes cases based on whether they can be prosecuted, and has not meddled in efforts to improve voting systems.
Although Justice Department probes into voter fraud are rare, it conducted a five year investigation during the George W. Bush administration. No evidence of fraud was found and the probe prompted more investigations and forced resignations of then-Attorney General Alberto Gonzales and seven U.S. attorneys.
Voting rights experts at New York University's Brennan Center for Justice recently urged the Justice Department to learn from its past.
"In 2007, the Justice Department was upended by scandal because it had pursued a partisan agenda on voting, under the guise of rooting out suspected 'voter fraud,'" wrote the center's Adam Gitlin and Wendy Weiser on January 7.
Any Justice Department probe could be led by Senator Jeff Sessions, the president's choice to lead the agency. Sessions has claimed in the past that voter fraud exists, but recently tried to separate himself from Trump's assertion that millions committed voter fraud.
Spicer cited Tuesday a 2008 Pew Research study that supposedly determined that 14 percent of U.S. voters were not citizens. Pew did not produce such a study, but the Washington Post released a disputed 2014 report that came to a similar conclusion.
Pew produced a 2012 study that found evidence of outdated voter registration forms but did not conclude there was actual fraud.