From NPR News, this is ALL THINGS CONSIDERED. I'm Audie Cornish. The stage is set in Denver for the first presidential debate tonight at 9:00 p.m. Eastern. The candidates are suiting up, reporters are gathering, live tweeters are sharpening their virtual pencils. And NPR's Mara Liasson is in Denver and she joins us for a preview. Welcome, Mara.
MARA LIASSON, BYLINE: Hi, Audie.
CORNISH: So what is the format for tonight's debate?