Seven of the Democratic presidential candidates are in Iowa Tuesday for a two-hour debate hosted by NPR, as an extremely close race is shaping up in the first-in-the-nation caucuses.With less than five weeks remaining before the Iowa caucuses, polls show that Illinois Sen. Barack Obama has closed the gap with Sen. Hillary Clinton (NY) - and former Sen. John Edwards (NC) is not far behind.The Democratic candidates will sit down with NPR hosts in Iowa on Tuesday for a two-hour debate. Tuesday's debate will give all of the Democrats a chance to sway those Iowa voters during an in-depth discussion. NPR hosts Steve Inskeep, Michele Norris, and Robert Siegel will moderate the debate, which will follow the same format introduced by NPR in 2004. There is no studio audience nor video cameras. Candidates will be able to engage one another in three areas of discussion, and field questions sent in by NPR listeners. Democratic presidential candidates clashed over Iran in the opening minutes of a radio debate Tuesday, broadcast live from Iowa by NPR and Iowa Public Radio.