One of the Senate's liberal leaders said Wednesday that it would be beneficial for President Obama to face a primary challenger from the left in 2012. 

Sen. Bernie SandersBernard (Bernie) SandersProgressive House Dem pushes for vote on 'Medicare for all' bill Castro takes steps toward likely 2020 bid Election Countdown: Lawsuits fly in Florida recount fight | Nelson pushes to extend deadline | Judge says Georgia county violated Civil Rights Act | Biden, Sanders lead 2020 Dem field in poll | Bloomberg to decide on 2020 by February MORE (I-Vt.), who has criticized the president for — among other things — brokering a tax deal with Republicans, said that it is worthwhile to have a wide array of voices in the campaign.

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"If a progressive Democrat were to run, I think it would enliven the debate," he said during an interview on WNYC Radio. "They are free to do that."

Sanders's comments could revive talk of a primary challenge to Obama; many observers speculated that could happen after Democrats took a beating in the midterm elections, making the president look vulnerable for reelection in 2012.

But Obama's poll numbers have rebounded since then, leading elections handicappers to say his reelection chances look stronger than they did three months ago. 

Still, Sanders said that a challenge would be a positive thing for the Democratic field.

"In a democracy, I think it's worthwhile to have different voices out there," he said.

Sanders was one of the politicians talked about as a potential primary challenger in December after he spent eight-and-a-half hours on the Senate floor railing against Obama's tax-cut deal. 

The self-described democratic socialist again ruled out a run Tuesday after he did so at the end of last year.

"I'm not a Democrat, I'm the longest-serving independent in Congress," he said.

A number of other potential candidates, including former Sen. Russ Feingold (D-Wis.) and Rep. Dennis Kucinich (D-Ohio), have also said they will not run.