With attention on recount efforts in several swing states, President-elect Donald Trump late Sunday claimed, without proof, that "millions of people...voted illegally."

Trump wrote on Twitter that "serious voter fraud in Virginia, New Hampshire, and California" cost him the popular vote, though current tabulations estimate that Democratic nominee Hillary Clinton is ahead by nearly 2 million votes.

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

— Donald J. Trump (@realDonaldTrump) November 27, 2016

During the rant, Trump argues that his campaign strategy of visiting many small states would have made it "much easier" for him to "win the so-called popular vote than the Electoral College," where he is currently ahead of Clinton 290-232.

"There's no reason to believe this is true," said Rick Hasen, blogger at the Election Law Blog and professor specializing in election law at the University of California, Irvine.

"The level of fraud in U.S. elections is quite low," Hasen noted, adding that, in particular, "the problem of non-citizen voting is quite small—like we're talking claims in the dozens, we're not talking voting in the millions, or the thousands, or even the hundred."

Fact-checking website Politifact also ruled Trump's assertion false.

While it is not the first time the president-elect has made unsubstantiated claims about the validity of the U.S. electoral process, this one came after it was announced that the Clinton campaign was joining the recount efforts spearheaded by Green Party leader Jill Stein. A recount is currently underway in Wisconsin, while Stein is expected to file in Pennsylvania and Michigan, as well, which have deadlines of Monday and Wednesday, respectively.

Noting the irony of Trump's claims, Clinton campaign attorney Marc E. Elias wrote on Twitter:

We are getting attacked for participating in a recount that we didn't ask for by the man who won election but thinks there was massive fraud

— Marc E. Elias (@marceelias) November 28, 2016

Beyond this latest tirade, observers are noting how the president-elect's use of Twitter to spread misinformation may have dangerous implications under a Trump presidency.

As T.C. Sottek, managing editor at The Verge wrote Sunday, "this is just one false claim of many" as Trump "has used Twitter to tweet and retweet false and misleading information at a volume that has challenged the bandwidth of fact checkers."

"When Trump becomes president," Sottek warned, "his Twitter account won't just be the ramblings of a private citizen—it will be the remarks of the chief executive of the U.S. government. And if his Twitter account is the most open part of his administration, the platform could effectively become the White House press office."