Caitlyn Jenner, perhaps the most influential transgender activist in the country, joined a growing chorus of critics who have denounced President Trump’s decision to rescind protections that allowed transgender students to use the bathroom of their choice.

Ms. Jenner, an Olympic gold medalist and reality television star, delivered a blunt warning to Mr. Trump in a video posted on Twitter on Thursday. “This is a disaster,” she said, adding, “See you in court.”

That statement echoed the president’s declaration on Twitter after his travel ban on immigrants from certain nations was blocked by the courts.

Ms. Jenner’s rebuke raised the level of opposition, and not only because of her high profile. She is a lifelong Republican and an early supporter of Mr. Trump’s when he was still thought to be a long shot to win the party’s nomination.