Former Green Party presidential candidate Ralph Nader is circulating a letter calling for a primary challenge to President Obama.

"In an uncontested Democratic primary, President Obama will never have to justify his decision to bail out Wall Street’s most profitable firms while failing to push for effective prosecution of the criminal behavior that triggered the recession, or his failure to push for real financial reform," Nader writes. "He will not have to defend his decision to extend the Bush era tax cuts nor justify his acquiescence to Republican extortion during the debt ceiling negotiations."

Other signatories to the letter include Princeton Professor Cornel West, a prominent African-American voice, former Sen. James Abourezk (D-S.D.), and writer Gore Vidal.

Nader is unpopular with many liberals because they view his 2000 candidacy as having peeled votes away from Al Gore's campaign in Florida, ultimately giving President George W. Bush the path to victory. He ran again in 2004 and 2008, that year as an independent.

An Obama challenger is unlikely to materialize. Still, the letter is further proof that some liberals are dissatisfied with the president.