The first debate between John McCain and Barack Obama, seen by an estimated 52.4 million viewers Friday, was one of the first events of the U.S. presidential campaign that was not a huge TV hit.

The audience was 10 million less than the first debate between President George W. Bush and Democratic challenger John Kerry in 2004, according to Nielsen Media Research on Monday.

It was not even one of the 10 most-watched presidential debates, falling well short of the record 80.6 million people who saw the only debate between President Jimmy Carter and Ronald Reagan in 1980, Nielsen said.

Nielsen's count includes 11 networks that televised the debate: ABC, CBS, Fox, NBC, CNN, Fox News Channel, MSNBC, CNBC, BBC America, Telemundo and Telefutura. Nielsen did not have estimates of the audience for the non-commercial PBS and C-SPAN.

Two factors may have held the audience down:

• It was not clear until midday Friday that the debate was even taking place; McCain earlier in the week had called for it to be postponed because of the financial crisis in the U.S. but changed his mind on Friday and agreed to participate.
• Friday traditionally has fewer viewers than any other night but Saturday because more people are going out.

Still, the 52.4 million would make it the most popular television event since the Super Bowl.

The debate between vice-presidential contenders Joe Biden and Sarah Palin is scheduled for Thursday, usually a big night for TV viewing. The most-watched vice-presidential debate ever was in 1984, when 56.7 million people watched vice-president George Bush take on Geraldine Ferraro, the first woman on a major party ticket.