The Democratic presidential debate held in Des Moines, Iowa, on December 13, was confined to only six candidates. They were Biden, Clinton, Dodd, Edwards, Obama and Richardson.
The debate sponsor, the Des Moines Register, required candidates to have had a campaign office open in Iowa, prior to October 1, 2007. Dennis Kucinich and Mike Gravel did not meet that requirement. This is the first Democratic presidential debate this year with fewer than seven participants. No Republican debate this year has had fewer than seven participants either, except for one that had only six (thanks to the commenter below). There have been 22 presidential debates this year for Republicans and Democrats.