Welcome to the Stephen Colbert primary

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.

Sen. Bernie Sanders sat down with Colbert one month ago. So did former HUD secretary Julián Castro. Sen. Kamala Harris was there last week. And Sen. Kirsten Gillibrand is expected to share some news about her presidential aspirations in an interview on Tuesday's show.

It will be Gillibrand's second appearance in just over two months. When she was on the "Late Show" two days after the midterms, she said she was giving "long hard thought" to a 2020 run.

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.

The anti-Trump bent of Colbert's "Late Show" helped the late-night host win more and more viewers. For the past two years, he has been the highest-rated host in late night. Lately he has also been challenging Jimmy Fallon in the key 18- to 49-year-old demographic, which is especially coveted by advertisers.

Colbert's interviews with politicians have not been a drag on the show's ratings. If anything, they've been a boon.

Aides for several of the most-talked-about contenders confirmed that they view Colbert as a crucial stop on the presidential roadshow.

They declined to speak about the booking process on the record, however.

"We strategize about Stephen a lot," a communications aide for a yet-to-be-declared candidate said, referring to Colbert on a first-name-basis.

Of course, the comedy route works better for some politicians than for others. A politician's comfort and charm in a late-night setting is considered when the show's bookers work with communications directors to schedule a guest.

But the list of Colbert guests since last summer tells the story. Eric Holder visited last July. Cory Booker in August. John Kerry, Beto O'Rourke and Hillary Clinton in September. Nancy Pelosi in October. Amy Klobuchar in November.

Harris made "The Late Show" one of the first stops on her book tour, which in and of itself is not surprising for an author.

What was surprising was the placement: Harris was the first guest, right after Colbert's monologue, a spot usually reserved for a comedian or Hollywood celebrity. Colbert and the producers let the interview run for two segments.

Vanity Fair's Chris Smith, who was backstage at the theater that day, wrote that Harris "smiled and exhaled" afterward.

"The senator seemed most proud of having gotten a laugh out of Colbert, a moment that happened after the TV cameras shut down," he wrote.

Colbert is said to be keenly aware of his power broker status with the Democratic electorate. He pays close attention to the Democratic field of candidates, sizing them up like so many others in the media business.

But Colbert also knows he is hosting a late-night talk show, not an MSNBC broadcast. He straddles both worlds -- joking with Rep. Adam Schiff one minute, asking about Russian espionage the next minute.

The path to the White House has involved late-night comedy pit stops for decades. Among political strategists, "Late Night with Seth Meyers," "Jimmy Kimmel Live" and "The Daily Show with Trevor Noah" are also seen as valuable bookings.

Julián Castro, who entered the 2020 race last weekend, sat down with Noah on Comedy Central last October.

Then he appeared on Colbert's show -- alongside his twin brother Rep. Joaquin Castro -- in mid-December. Julián was there to promote a book. He wouldn't say whether he was going to run for president, but Joaquin predicted that he would.

"I'll speak on his behalf: He's going to run for president," Joaquin said, sparking cheers from the crowd.