Television’s Trump bump may be fading. Or perhaps the reticent witness — a career prosecutor who delivered terse, technical answers — was not the type to keep Americans tuned in for a marathon day of viewing.

Whatever the reason, the ratings for Robert S. Mueller III’s congressional testimony on Wednesday failed to match the big viewership for other recent political spectacles.

An average of 13 million Americans watched the former special counsel on the major cable and broadcast networks over the seven-and-a-half hours of questioning, according to statistics released on Thursday by Nielsen.

That audience was smaller than the 19.5 million people who watched James B. Comey, the former F.B.I. director, describe his dealings with President Trump to Congress in June 2017. Mr. Comey proved a surprising and animated witness, offering memorable one-liners (“Lordy, I hope there are tapes”) that attracted about the same audience as Game 2 of that year’s N.B.A. finals.