Mayor Bill de Blasio lashed out at the commander-in-tweet on Monday, claiming Donald Trump is telling a “dangerous lie” in making baseless claims of voter fraud.

The mayor said he feared Trump and Republicans could use the false accusations to suppress future votes.

“Dangerous LIE that if allowed to spread will be basis for Republican effort to restrict voting access even further,” de Blasio wrote in retweeting Trump’s conspiracy theory posted Sunday. “We can’t let that happen.”

Trump won the Electoral College but will lose the national popular vote to Democrat Hillary Clinton by at least 2 million ballots — and north of 1.5 percentage points.

Trump said Clinton’s popular victory was based solely on ballots illegally cast for her.

The president-elect singled out California, Virginia and New Hampshire as the prime suspects for illegal voting but offered no proof to back up those wild claims.