The matchup in Nashville, which featured a relatively subdued president trying to revive a flagging candidacy, ranked 17th on the debate ratings chart since 1976, when Nielsen began measuring total viewership numbers for the events. It narrowly beat the 62.7 million who watched the final debate between Jimmy Carter and Gerald Ford in 1976, 44 years ago to the day from Thursday’s debate.

The ratings drop on Thursday could stem from many causes: viewer backlash to the raucous first Biden-Trump debate, competition from an N.F.L. game, or attrition from a marathon-length presidential race nearing the finish line. (The first debate of the Democratic primary took place in June 2019.)

Still, Mr. Trump is coming off another somewhat ominous ratings milestone: his unexpected Nielsen defeat last week to Mr. Biden in a battle of dueling network town-hall-style events. Mr. Trump lost in the ratings although his event was shown on NBC, CNBC and MSNBC, while Mr. Biden’s appeared only on the ABC network.