 -- President-elect Donald Trump made baseless claims Sunday that "millions of people" voted illegally for Hillary Clinton, in turn costing him the popular vote -- but his transition team provided no evidence to back up the allegations when pressed today.

"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," Trump tweeted. Clinton currently leads the national popular vote by more than 2 million votes.

In another tweet, he named Virginia, New Hampshire and California as three states with "serious voter fraud." There is no evidence to back up this claim, and it's unclear as to why these states were selected.

When asked for proof of his claims, the Trump transition team did not point to evidence of voter fraud in this election, instead citing a 2012 Pew Research Study on the need to update voter registrations, and a controversial Old Dominion University study posted in 2014 by The Washington Post that found that 6.4 percent of non-citizens voted in 2008, and 2.2 percent in 2010.

These two studies are not linked to the recent election on Nov. 8. Still, Trump had also cited these studies on the campaign trail to make flawed claims of voter fraud and rigged elections.

"Then there's the issue of illegal immigrants voting. The following comes from a 2014 report from the Washington Post. This article was entitled, could non-citizens decide the November elections. Here are some excerpts. More than 14 percent of non-citizens in both 2008 and 2010 samples indicated that they were registered to vote. Oh, isn't that wonderful?" Trump said at an Oct. 17 rally in Green Bay, Wisconsin. "Because non-citizens tend to favor Democrats to put it mildly, Obama won more than 80 percent of the vote of non-citizens in the 2008 sample. You don't read about this right?"

And one day after the final presidential debate, Trump continued his claims.

"According to Pew, highly respected, there are 24 million voter registrations in the United States that are either invalid or significantly inaccurate," Trump said on Oct. 20 at an Ohio rally. "I think the people in this room understand it more than our leaders. Although, maybe our leaders understand more than we think they understand. Listen to this. 1.8 million people are dead. But they are registered to vote, some of whom vote even though they are dead, which is really a hard thing to do."

And this morning, Trump transition communications director Jason Miller cited the two studies as he slammed Green Party candidate Jill Stein’s recount efforts. He told reporters on a conference call, "If this much attention and oxygen is going to be given to a completely frivolous throwaway fundraising scheme by someone like Jill Stein then there should be actual substantive looks at the overall examples of voter fraud and illegal immigration voting in recent years."

But since Trump began using these studies to argue his rigged election claims, the authors of the two studies have made clear their studies have not shown evidence of widespread voter fraud, as Trump has alleged.

The primary author of the Pew study, David Becker tweeted today: "As primary author of the report the Trump camp cited today, I can confirm the report made no findings re: voter fraud. We found millions of out of date registration records due to people moving or dying, but found no evidence that voter fraud resulted. Voter lists are much more accurate now than when we issued that study in 2012, thanks to the 20 states sharing data through @ericstates_info."

And Jesse Richman, the co-author of the Old Dominion study, shared a post on his website in October, saying he stands by the study, but the results of the study have been "misread."

"On the right there has been a tendency to misread our results as proof of massive voter fraud, which we don't think there are," he wrote. "Our focus has been on the data rather than the politics."

When pressed further Monday, Miller did not provide evidence of voter fraud this election or any evidence of Trump's claims of voter fraud in Virginia, New Hampshire and California.

Former Virginia Solicitor General and Republican election law attorney William Hurd told ABC News that Trump is likely misled, noting that even if there were problems, they would not have resulted in Clinton's carrying the state of Virginia.

"Virginia has a strong bi-partisan tradition of running clean elections. If our President-elect believes there has been ‘serious voter fraud’ here, we all want to hear about it. But, it is far more likely that he has been misled," said Hurd, who led three statewide recounts in Virginia. "And, to the extent there may have been any problems, that is not why Clinton carried the state.”

In none of the tweets did Trump himself point to any evidence of voter fraud to account for his claim, and no evidence of his assertion has emerged since the election.