The title of "An Unreasonable Man," an improbably riveting portrait of Ralph Nader, is from George Bernard Shaw, who wrote: "The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man."

I guess "Inconvenient" was taken.

Choosing the Shaw quote as the film's opening epigram speaks volumes about the filmmakers' perspective on Nader, who for millions of Democrats immolated a heroic legacy of consumer activism and advocacy by running for president in 2000 -- thus, in their view, contributing to the election of George W. Bush.

That issue is tackled head-on at the top of "An Unreasonable Man," setting up the narrative tension of a film that then doubles back to document Nader's epic battle with the auto industry over safety issues in the 1960s and the juggernaut of consumer consciousness and policy shifts he so effectively piloted thereafter. Using a combination of archival footage and talking heads, directors Henriette Mantel and Steve Skrovan also chronicle how Nader came to make that quixotic 2000 run.

Some of the most fascinating passages of "An Unreasonable Man" have to do with Nader's early years in Washington, when General Motors, alarmed at his skill and zeal, had him followed, with an idea of mounting a smear campaign. At one point they sent an attractive young woman to Nader's neighborhood Safeway to seduce him. It's risible now, but it's a chilling reminder that political payback is beanbag compared with what happens when you threaten corporate profit.

"An Unreasonable Man" tries hard to be a balanced, warts-and-all portrait, the warts being puritanical righteousness, zero people skills and an office whose disorganization reflects managerial weaknesses. But in the final analysis, it tips just slightly into hagiography, as when Mantel -- a former Nader staffer -- gets misty-eyed while speculating about about his love life (ick). Still, the list of Nader's accomplishments -- from seat belts in cars to his crucial advocacy for public health, consumer protection and environmental issues -- reminds us of what should have been his most enduring legacy.

When it comes to 2000, the filmmakers toggle back and forth between disgruntled Dems and Nader apologists, finally enlisting a Harvard government professor to analyze Nader's campaign spending and appearances just before the election to ascertain whether he set out to be a spoiler. Verdict: Ralph's clean. Leaving aside persistent accusations of intellectual dishonesty during that campaign, a less pointed question is left unanswered: Just how effective have Nader's 2000 and 2004 presidential runs been in building third parties or effecting real change in the electoral landscape?

Nader haters may not be mollified, and the pain of "Raiders" who felt betrayed by their idol's decision to run -- the result of a mixture of political purism and personal pique -- is palpable and surprisingly moving. "An Unreasonable Man" crackles with fascinating politics and passion even today; like its subject, it's a one-stop civics lesson -- one that shouldn't be missed.

An Unreasonable Man(122 minutes, at Landmark's E Street Cinema) is not rated. It contains three swear words, none of them uttered by Nader.