And for all their differences, Sanders and Biden have a mutual interest in preserving their duopoly and using each other as foils. Sanders needs an establishment antagonist, while Biden, 76, may prefer running against the 77-year-old Sanders than 20 younger options.

“With Biden in the race, it gives (Sanders) someone to contrast with,” said Mark Longabaugh, who was a aide in Sanders’ 2016 campaign. “I still think one of Bernie Sanders’ central challenges is being able to adjust to a front-runner’s position and ultimately building a coalition that makes him the nominee.”

Longabaugh added: “Biden has the opposite challenge. He has the ability to build a broad coalition, but his challenge is being able to consolidate really core support and enthusiasm. It seems that a lot of people are for Biden, but are they really for Biden?”