CNN has announced eligibility criteria for the first Democratic presidential debate, to be held October 13 in Las Vegas. In addition to Hillary Clinton and Bernie Sanders, 3 other candidates meet a minimal threshold polling requirement: Martin O'Malley, Lincoln Chafee and Jim Webb. 

Vice president Joe Biden has met the polling criteria; he will be able to participate in the debate if he announces his intention to run as late as the day of the event. 

This is one of six debates sanctioned by the Democratic National Committee, a limited schedule that has caused some controversy. The next two debates will be in the first two primary/caucus states. The first will be November 14 in Des Moines, followed by a December 19 event in Manchester, New Hampshire.