When Jimmy Kimmel dropped the humor and began a relentless campaign against the latest effort from Senate Republicans to repeal the Affordable Care Act, it was — at the very least — an uncommon moment for him.

While Mr. Kimmel often discusses politics on his show, it is not his main focus and he has rarely — if ever — taken such a serious tone to advocate for or against a single piece of legislation. In the Trump era, late-night hosts speaking out against President Trump are common. Leaving out the jokes, however, is not.