Near the end of “An Unreasonable Man,” a sympathetic but not uncritical documentary portrait of Ralph Nader by the directors Henriette Mantel — a former protégé — and Steve Skrovan, the film’s subject allows himself the bitter pleasure of joining his fellow left-wingers in what has now become the cliché of wondering whether George W. Bush is “the worst president ever.”

Until then, Mr. Nader’s stubborn refusal to take responsibility for Mr. Bush’s election victory in 2000, when he split the progressive vote as the Green Party candidate, made perfect sense. If you accept the Naderite view that the two major parties are increasingly indistinguishable, then the value of his offering the electorate a real choice must far outweigh any trivial differences there might have been between a Gore and a Bush presidency.

But now here was the man himself telling us that, in effect, the barbs of his Democratic critics — whose vitriol directed at him appears here at times to be even greater than that directed at the president — were justified all along.

For if the Bush presidency is as bad as he says it is, doesn’t that mean that there are important differences between the parties? How can he go on justifying his failure to back the Democrats — if not Mr. Gore, since he was then ignorant of how bad Mr. Bush would be, then at least John Kerry in 2004?

The critics would say, as some of them do in this film, that the reason was pure vanity. I don’t believe this. Everything we see of the crusading “public citizen” and consumer advocate in the 30-odd years leading up to 2000 that occupy most of “An Unreasonable Man” suggests that vanity and personal ambition are not among his character flaws, if any such flaws exist.

But it is also true that Mr. Nader could be regarded as the progenitor of his critics, and not only because many of them got their start in politics as “Nader’s Raiders” in the 1970s and ’80s. They, like their mentor, are creatures of the same1960s-era belief in politics as a struggle between the forces of light and darkness — the same belief that now casts Mr. Nader as the Prince of Darkness. There is a kind of poetic justice to it. You begin by making a devil of General Motors, and you end by being made a devil yourself.

Yet Mr. Nader had an important point to make about the ill consequences that have ensued since politics became a branch of marketing, and “An Unreasonable Man” does a good job of letting it emerge.

Ms. Mantel and Mr. Skrovan trace the change to the influence of former congressman Tony Coelho on the Democrats in the 1980s, when the party of the little guy learned how to go after the big corporate contributors, whose largesse had hitherto gone mostly to Republicans. The directors could have looked a lot further back than that, but the point remains that when politics becomes an attempt to win market share, the two major parties have to come ever closer together to fight for the same few swing voters in the middle.

As a result, political “debates” — including the Bush-Gore debates in 2000, whose exclusion of Mr. Nader is the subject of some of the most interesting passages in the film — are reduced to dueling platitudes. Moreover, the shrinking of substantive differences between the parties means that trivial and personal ones become exaggerated. Making a market for your political product is accomplished by creating differences in contemptible ways — that is, by vicious attacks on an opponent’s character and fitness for office rather than his policies.

We’ve seen this brought to a new height in the last two elections, which the Democrats have sought to make turn on Mr. Bush’s personal shortcomings. Without a coherent policy of their own on the Iraq war, they have treated the main foreign policy issue of the day only as a source of examples of the President’s “lies,” incompetence, or stupidity. Thus, when John Kerry was asked in 2004 what he would do differently about Iraq, his answer was: “Everything!”

That such a ludicrous evasion can now pass for political seriousness is exactly what the candidacy of Ralph Nader should have pointed up. But though he has been admirably forthright about the war himself, he also changes the subject by indulging in what Bill Clinton called “the politics of personal destruction” when it suits him to do so.

Probably, he cannot do otherwise. This, after all, is the man who went after the meatpacking industry by calling hot dogs “missiles of death.” It’s not exactly his fault that such hyperbole has become the common political currency, but neither is it entirely inappropriate that he has now become its victim.

jbowman@nysun.com