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 before.

Nader is running for the same reason he has run in the past: Because the likely nominees of the two major parties do not begin 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 the Democratic nominee (be he Barack Obama or be she Hillary Clinton) or by Republican John McCain. And that, says Nader, will leave millions of Americans feeling frustrated and disenfranchised.

“You take that framework of people feeling locked out, shut out, marginalized and disrespected,” he explained on NBC’s “Meet the Press,” the same forum where he announced his 2004 presidential run. “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.”

Nader’s points are all well taken.

And they come from a man who is quite rational in his awareness that he will not be sworn in as president on January 20, 2009.

While Nader has yet to determine whether he will run as the Green Party candidate, a Green-backed independent or a genuinely unaffiliated independent, he is clear about his chances.

The arc of history bends toward Obama and the Democrats, not his candidacy, acknowledges Nader.

After eight years of George Bush and Dick Cheney, he said, “If the Democrats can’t landslide the Republicans this year, they ought to just wrap up, close down, emerge in a different form. You think the American people are going to vote for a pro-war John McCain who almost gives an indication he’s the candidate for perpetual war?”

Presumably, the Democratic landslide that buries McCain will also sweep away various and sundry third-party and independent candidacies, including Nader’s.

If that is the case, it will not be a new phenomenon.

Nader has bid for the presidency in different ways in every election since 1992 — as a write-in candidate in the New Hampshire and Massachusetts primaries of that year, as a Green contender in 1996 and 2000 and as an independent with support from some of what remained of Ross Perot’s Reform Party in 2004. His most notable run, in 2000, won 2.7 percent of the national vote, along with anger from Democrats who thought he “spoiled” their chances by tipping Florida — and the presidency — from Al Gore to George Bush. In fact, Gore won Florida, only to have the results manipulated into Bush’s column by the Republican nominee’s many allies in state government, with an assist from the Supreme Court.

In the intense 2004 competition between Bush and Democratic John Kerry, Nader’s run won just 0.3 percent on 34 state ballot lines.

This year, Nader could have a harder time of it even than he did in 2000 or 2004.

Unlike Gore and Kerry, Obama — now the likely Democratic nominee — has taken savvier stands on a number of issues close to Nader’s heart, such as trade policy. This is not to say that Obama is as good as Nader on the issues. Far from it. But 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 something less than a pristine progressive.

Obama may be “the first liberal evangelist in a long time,” says Nader, but the senator’s “better instincts and knowledge have been censored” since he hit the nation stage.

“(Obama’s) leaned, if anything, toward the pro-corporate side of policy-making,” Nader said of the senator from Illinois. The consumer activist also scored Obama on on foreign policy, noting that, “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).”

Such blunt statements may not win Nader many friends among Obama’s enthusiastic backers, and Obama did not exactly welcome his new rival to the race. “Ralph Nader deserves enormous credit for the work he did as a consumer advocate,” Mr. Obama said while campaigning in Ohio “But his function as a perennial candidate is not putting food on the table of workers.”

But Nader’s not looking for Valentines from the Democrats.

Frankly, he’s not even all that interested in popular approval.

The public-interest crusader worries far less about poll numbers and even 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 — like Socialist Norman Thomas in his races against Democratic Franklin Roosevelt — as a source of pressure on the Democratic nominee to address fundamental questions and perhaps to take more progressive stands on a few issues. As in 2000 and 2004, Nader’s appeal will be determined in large part by the extent to which the Democratic candidate is willing to be bold.

Obama seems to understands this. Unlike Gore or Kerry, who never quite “got” the point of Nader’s runs in 2000 and 2004, the Illinois senator appears to recognize that it is pointless to grumble about Ralph 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.

If Obama runs as a progressive, Nader will have little room to maneuver. If Obama runs to the center, Nader’s space will open up — a bit.