So my suggestion is, I think one of the reasons the president has been able to move so far to the right is that there is no primary opposition to him and I think it would do this country a good deal of service if people started thinking about candidates out there to begin contrasting what is a progressive agenda as opposed to what Obama is doing."
Guys like Bernie Sanders want a full-throated Marxist in the White House. Someone who will come right out and say that they want to abolish the US Constitution, and supplant it with a totalitarian, Communist government.
Sanders and his ilk are upset that Obama hasn't taken us further toward their Socialist Utopia than he already has. They're not at all worried that they might lose power in Congress and the White House because he's veered too far left.
If he thinks the way to success is in forcing Obama even harder left, he hasn't been paying attention to the polls and the most recent elections.