Sen. Kirsten Gillibrand is expected to announce she's running for president when she visits Colbert's Late Show on Tuesday. "Welcome to the Stephen Colbert primary," says CNN's Brian Stelter. He notes that in recent months Colbert has hosted Democrats with presidential aspirations, including Sen. Bernie Sanders and former HUD secretary Julián Castro last month, and Sen. Kamala Harris last week. Promoting her new memoir, Harris was given the No. 1 guest slot last Thursday, a spot usually reserved for a celebrity that typically runs for two segments. "Iowa. New Hampshire. Donor enclaves on the coasts. And... the Ed Sullivan Theater?" says Stelter. "Yes, the home of Stephen Colbert's Late Show is proving to be a key stop for Democrats who are positioning themselves for 2020 presidential bids." Stelter adds: "Colbert's studio is a logical place for Democrats to tease presidential campaign plans. He has a liberal fan base dating back to his days on Comedy Central, and that audience crystallized after Colbert took aim at then-candidate Donald Trump in 2016...Colbert's interviews with politicians have not been a drag on the show's ratings. If anything, they've been a boon."