Speaking on CNN’s State of the Union on Sunday, Biden said that he would fight for the presidential nomination at the Democratic National Convention in Milwaukee in July if Sanders leads in the delegate count but does not have at least 1,991 pledged delegates, a number that constitutes a majority.

“The rules have been set,” Biden said. “You don’t change the rules in the middle of the game.”

With analysts beginning to argue that it is unlikely any candidate will have a majority of delegates before the convention, Sanders has begun to challenge these rules.

At a CNN town hall in late February, for instance, Sanders said, “If I or anybody else goes into the Democratic convention with a substantial plurality, I believe that individual, me or anyone else, should be the candidate of the Democratic Party.”