The lead-off presidential debate between Sens. John McCain and Barack Obama on Friday was seen by an estimated 52.4 million viewers.

That may sound like a lot of viewers, but according to Nielsen Media Research, the audience was 10 million less than the first debate between President George W. Bush and his Democratic challenger, Sen. John Kerry, in 2004.

It was also well short of the record 80.6 million people who saw the only debate between Democratic President Jimmy Carter and Republican challenger Ronald Reagan in 1980.

Nielsen, whose count included 11 networks that televised the debate, said the Obama-McCain debate wasn't even one of the 10 most-watched presidential debates of all time.

Two factors may have held the audience down, Nielsen said: It wasn't clear until midday Friday that the debate was even taking place; and Friday typically has fewer viewers than any other night except Saturday because more people are going out.