Obama told a group of Democrat donors last week that Sanders is coming to the point where his campaign to become the nominee will end and that the party should unite behind former secretary of state Clinton, the New York Times reported.
But Sanders defied suggestions his campaign was coming to a natural conclusion.
“The bottom line is that when only half of the American people have participated in the political process … I think it is absurd for anybody to suggest that those people not have a right to cast a vote,” Sanders told MSNBC.
It comes as Clinton dominates the contest between the two Democrats, and remains the frontrunner to be the next President.
Meanwhile, on the Republican side of the race Donald Trump looks set to take the nomination, leading to a run off between the billionaire and Clinton.