WASHINGTON — When Donald J. Trump was running for president, he promised to “open up our libel laws.” No one quite knew what he meant.

Last month, Mr. Trump started to make good on his campaign pledge. But he did so inadvertently — as a libel defendant on the losing side of a decision that will help victims of sexual misconduct sue when they are called liars.

“The irony is glaring,” said Roy Gutterman, who teaches communications law and journalism at Syracuse University.

Before the #MeToo movement, libel lawsuits from people accused of lying were in decline. Indeed, a 2016 law review article chronicled what it called “the slow, quiet and troubled demise of liar libel.”