The Nielsen company said the debate, seen by 19.7 million people on NBC or MSNBC on Wednesday, attracted more viewers than any Democratic nomination contest ever.
It was more than double the 7.9 million people who watched ABC's coverage of a debate just before the New Hampshire primary on Feb. 7. It beat the 18.1 million people who tuned in for the second night of the first debate in this campaign season, back in June.
The session's first half hour was particularly action-packed.
The ratings also indicate that with Super Tuesday primaries approaching next month, voters are starting to pay closer attention.
NBC said the debate attracted an additional digital audience of 417,000 viewers.