Liberals like Sanders have been critical of Obama for engaging with Republicans in negotiations over how much to cut spending, in connection with an agreement to raise the nation's debt ceiling. Sanders in particular has been a vocal opponent of any plans to transform entitlement programs like Social Security and Medicare.
Sanders made no indication that he would be the one to wage a challenge to the president; he faces his own reelection battle in Vermont in 2012.
It's not clear whether anyone else in the Democratic Party would have the stomach for taking on an incumbent president, one who faces a tough enough reelection without having to divert resources toward fending off a challenge from his left. Former Sen. Russ Feingold (D-Wis.) had been a subject of speculation, but his spokesmen have said he supports Obama's reelection.