Trump has crossed a new line with murder accusations and could be sued while president for defamation and "wantonly cruel attacks," Yale law professor writes.
The lawsuit, which takes aim at two opinion essays related to Russian election interference, comes on the heels of a similar suit against The New York Times.
The longtime advice columnist blamed her firing on the president, who she said "dragged me through the mud."
The 2016 Democratic nominee is accused of spreading lies about Gabbard to harm her 2020 presidential campaign.
Last summer, the columnist accused Trump of raping her in a department store dressing room in the 1990s. He responded by saying he'd never even met her.
The Democratic presidential contender's lawyers demanded that Clinton issue a retraction for calling an unnamed candidate "the favorite of the Russians."
The president blamed "the LameStream Media," saying that he didn't want the Texas congressman to undergo "months of slander and libel."
The actor claimed as part of his defamation lawsuit that she "hit, punched and kicked" him.
Daniels had sued Trump for defamation earlier this year, but a judge ruled the president was protected by the First Amendment when he alluded she was a liar.