It was the biggest political clash in American history — by one measure, at least.

The first presidential debate between Hillary Clinton and Donald J. Trump was watched by 84 million television viewers, Nielsen said on Tuesday, the largest audience since televised debates began in 1960.

In an era when mass events are increasingly rare, Monday’s meeting of the candidates was seen in nearly half of American households with television sets. The viewership toppled the previous debate record of 80.6 million, set during the sole 1980 debate between President Jimmy Carter and Ronald Reagan in the days before social media and cable TV.

The tally did not include Americans who watched Monday’s debate on the internet or on mobile devices. It also did not include viewers of C-Span, which is not rated by Nielsen.