Ralph Nader is running again for president. After four previous bids, mounted in varying forums and with varying goals, Nader is used to the slings and arrows that will be tossed his way. He is conscious and committed. He will not back off.

He knows how to campaign in the face of a firestorm of criticism. Above all, he knows how to make himself heard – even when almost everyone who guides the political processes of the nation wants to shut him up. The latter knowledge will serve him well in a 2008 contest where the man, who is either a national treasure or a national frustration, or perhaps both, may find himself more marginalized than ever.

Nader is running for the same reason as in the past: Because the likely nominees of the two major parties do not to meet the standards that might reasonably be asked of progressive contenders in 21st-century America.

Fundamental issues – Wall Street-defined globalization, rampant and frequently deadly corporate crime, out-of-control military spending and an imperial foreign policy – are not going to be addressed in a realistic let alone definitional manner by either Democratic or Republican presidential candidates. That, says Nader, will leave millions of Americans feeling frustrated and disenfranchised.

NADER IS A DETERMINED, SOMETIMES UNRELENTING, TRUTH TELLER.

“You take that framework of people feeling locked out, shut out, marginalized and disrespected,” he explained on NBC’s “Meet the Press. “You go from Iraq, to Palestine to Israel, from Enron to Wall Street, from Katrina to the bumbling of the Bush administration, to the complicity of the Democrats in not stopping him on the war, stopping him on the tax cuts.”

If he is the Democratic nominee, Obama’s more nuanced platform, as well as the movement character of the Illinois senator’s campaign, is likely to leave even less space for Nader to deliver a message. That said, Nader is a determined, sometimes unrelenting, truth teller. He notes that Obama is less than a pristine progressive.

“[Obama’s] leaned toward the pro-corporate side of policy-making,” Nader said of the Illinois senator. The consumer activist also scored Obama on foreign policy, noting, “He was pro-Palestinian when he was in Illinois… Now he’s supporting [right-wing Israeli policies that thwart progress toward peace in the Middle East].”

Nader’s not looking for Valentines from the Democrats. He’s not even all that interested in popular approval. The public-interest crusader worries far less about poll numbers and vote totals than about saying what he feels needs to be said – and using the forum of the electoral process to say it. And he is certainly not the first progressive, inside the Democratic Party or out, to suggest that Obama needs to be prodded on issues ranging from labor law to corporate regulation to single-payer health care and Middle East policy.

Nader’s greatest value in any race is as a source of pressure on the Democratic nominee to address fundamental questions and take more progressive stands on a few issues. His appeal will be determined by the extent to which the Democratic candidate is willing to be bold.

Obama appears to recognize that it is pointless to grumble about Nader as a “spoiler.” Rather, the point is to be more appealing to progressive voters who might consider voting Green or independent.

“I think the job of the Democratic Party is to be so compelling that a few percentage [points] of the vote going to another candidate is not going to make any difference,” says Obama. That is the bottom line with regard to Nader’s latest bid.

JOHN NICHOLS IS WASHINGTON CORRESPONDENT FOR THE NATION MAGAZINE.