The plaintiff in the lawsuit, Summer Zervos, is a former contestant on Mr. Trump’s show “The Apprentice.” In her complaint, she said Mr. Trump defamed her during the 2016 presidential campaign by repeatedly describing her accounts, and those of other accusers, as “total lies” and “made-up nonsense to steal the election” — potentially for fame or financial gain.

Besides arguing that the president could not be sued in state court under the Supremacy Clause, Mr. Trump’s lawyer, Marc E. Kasowitz, had argued that Mr. Trump’s comments about the women, made during a hard-fought election, amounted to political speech protected under the First Amendment. He also had argued that Ms. Zervos was not held up to the level of ridicule or contempt necessary to prove defamation.

Justice Schecter disagreed. She said Mr. Trump’s comments about Ms. Zervos telling “phony stories” could be construed as defamatory, suggesting she “is contemptible because she ‘fabricated’ events for personal gain.”

Ms. Zervos’s suit was being closely watched by other women who came forward during the presidential race with accusations against Mr. Trump. Some have said they would be willing to give depositions in the case.

During the campaign, more than 10 women made allegations against Mr. Trump ranging from unwanted touching to sexual assault. Most of them spoke out after the release of an “Access Hollywood” recording on which he bragged about kissing and groping women without their consent.