Vermont Senator Bernie Sanders continues to argue that a Democratic primary challenge to President Obama would be “good for democracy and for the Democratic Party.”

Sanders will not be a candidate. The Vermont independent, who caucuses with Senate Democrats, is running for re-election in 2012.

But Sanders, who has been sharply critical of Obama’s compromises with the Republican right on economic and fiscal policy, continues to talk up the idea of a primary challenge as a vehicle to pressure the president from the left. He is not alone. Ralph Nader is actively encouraging a primary race. And one-third of Democrats and Democratic-leaning independents tell pollsters that they favor a primary challenge to the president, while just 59 percent oppose such a run.

Referencing his regular appearances on Thom Hartmann’s nationally syndicated radio show, Sanders said: “I do a radio show every week. Over a million people hear it in almost every state in the country. Those are working-class people, progressive people. There is a lot of disillusionment. They want the president to stand up for the middle class, for the working class of this country, and they want him to take on big money interests in a way that he has not done up to this point.”

Who might challenge Obama? Sanders isn’t naming names. But in an appearance on C-SPAN’s Newsmakers program that was taped Friday, Sanders said: “I am sure there are serious and smart people out there who can do it,”

That’s an optimistic take. In fact, potential challengers have been reluctant to step up.

Critics of a primary challenge fear that it would not snatch the nomination from Obama but would weaken him in fall competition with a Republican such as Texas Governor Rick Perry or former Massachusetts Governor Mitt Romney.

But Sanders says: “Here’s the point: If you’re asking me, do I think, at the end of the day, that Barack Obama is going to be the Democratic candidate for president in 2012? I do. But do I believe that it is a good idea for our democracy and for the Democratic Party—I speak, by the way, as an independent—that people start asking the president some hard questions about why he said one thing during his previous campaign, and is doing another thing today on Social Security, on Medicare. I think it is important that that discussion take place.”