The so-called "overnights" are subject to change later in the day on Tuesday.
But the data indicates that Monday's face-off may well have been the most-watched debate in American history.
CNN and other cable news channels saw big increases over past election years. So did some of the broadcast networks.
The early Nielsen data confirms that viewership stayed high the entire time -- there wasn't a big drop-off after the first hour of the nearly 98-minute debate.
The debate was carried on more than a dozen TV channels, some big and some small.
NBC had the highest household rating overall, partly because "NBC Nightly News" anchor Lester Holt was the moderator of the debate.
Actual viewership figures will be released by Nielsen on Tuesday afternoon.
The viewership number to beat is 81 million, set back in 1980, when Jimmy Carter and Ronald Reagan debated just once before the election.
More recently, the first Obama-Romney debate in 2012 averaged 67 million viewers.
Nielsen's data only counts viewers who watched via traditional TV at home. It does not count viewing parties, bars, restaurants or offices.
Many millions also watched the debate via the Internet in ways that are not measured by Nielsen.
Various live streams on YouTube together registered more than 2.5 million simultaneous viewers.
Anticipation for the Clinton-Trump meeting had been mounting for months. On Monday night Twitter said it was the "most tweeted debate ever."
On both Twitter and Facebook, Trump was a livelier subject than Clinton. Twitter said the "final share of conversation around the candidates on stage" was 62% for Trump and 38% for Clinton.
On Facebook, the results were even more lopsided, with Trump earning 79% share of conversation and Clinton having the remaining 21%.
Being talked about isn't necessarily a good thing for a candidate. Most commentators gave Clinton the edge over Trump after the debate.
Still, the post-debate coverage on TV focused on Trump, partly because of surprising and confounding comments he made on stage.
CNN's reality check team investigated 26 claims made by Clinton and Trump and found that Trump made a greater number of misleading statements.