I suppose Dennis Kucinich is available (although there are many on the left who don't think he's quite liberal enough), but the question isn't whether they want to primary the president, the question is who could they get?
Via Hot Air, Bernie Sanders:
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. [...] So I would say to Ryan [sic] discouragement is not an option. I think it would be a good idea if President Obama faced some primary opposition.
Jesse Jackson is too "old news" and too tainted. Ditto Al Sharpton. There are no really prominent lawmakers in the Black Caucus. Ralph Nader hates the Dems as much as he hates the GOP. In short, there simply isn't a far left, loony toons liberal with a high enough profile to satisfy the Bernie Sanders wing of the Democratic party and could be made to look like a credible alternative to Obama.
How about Bernie himself? He would be the first openly socialist candidate to make a serious run for president since Norman Thomas in 1948. He would certainly be entertaining to watch. And it would be a nice ego trip for him. But Sanders cherishes the (I) after his name and is unlikely to join the Democratic party just to travel the country telling people that Obama is too conservative.
Who'd believe him anyway?