(CNN) -- Sean Penn and Gus Van Sant have a proposition for us: a biopic dedicated to the memory of San Francisco activist and City Supervisor Harvey Milk, the first openly gay man to be elected to public office in California, who was murdered by a fellow supervisor in 1978.

Sean Penn plays San Francisco City Supervisor Harvey Milk in "Milk," directed by Gus Van Sant.

The subject may be a tricky sell, but the timing feels right -- a few weeks late to save Californians' same-sex marriage rights, admittedly, but the need to keep on fighting through adversity may be Milk's most important legacy.

And "Milk" is a powerful movie that will stir more than a few hearts and minds.

An audiotape Milk records "just in case" is screenwriter Dustin Lance Black's convenient structuring device, foregrounding the story's tragic outcome and allowing Milk to narrate his own life story.

In this telling, it's a life that begins at 40 -- when he picks up Scott (James Franco), falls in love, comes out and drops out. The year is 1970, and San Francisco beckons. Their Castro Street camera store soon becomes a focal point for the booming gay community, and it's not long before Milk makes the first of several unsuccessful runs for district supervisor.

Civic elections might seem like small beer, but the persecution that compelled Milk to run for office is no trivial matter. The gay rights movement's most critical accomplishment, the film suggests, is how it liberated gays to be themselves.

As Milk tries to explain to his heterosexual colleague Dan White (Josh Brolin), this isn't about principles, it's about people's lives -- three of his lovers had threatened suicide. One of them, Jack Lira (Diego Luna), goes through with it. The political can't get more personal than that. Ironically, the devoutly "normal" White is the one who is truly messed up.

Here's another irony: To earn the recognition and validation of the voters, Milk has to shed his reborn hippie uniform and ponytail, put on a suit and get a haircut. Making the same calculation, director Gus Van Sant has axed the long takes and experimentalism that made "Elephant" and "Paranoid Park" arresting but decidedly marginal experiences and turned in his most conventional movie since "Finding Forrester." In other words, he's playing it straight this time.

The strategy is sound; the execution, nearly flawless. Van Sant captures the time and the place with unobtrusive precision, seamlessly mixing in reams of archival news reports. (She may not know it, but Anita Bryant has a co-starring role in this movie.) iReport.com: Share your reviews of 'Milk'

Penn is studied and thoughtful, impassioned and immediately sympathetic as Milk. It's easy to see how he attracts so much support -- and how his drive and commitment don't leave enough time for a "real" life. When Penn smiles, there's always pain there -- it's almost a wince -- and he smiles a lot here.

"Milk" may be a little too homogenized for some tastes. Like "Philadelphia" and "Brokeback Mountain," it's careful how it advances its agenda (and it does have one).

But it's not just a single-issue movie. In its conviction that "change" isn't effected through rhetoric alone, but through the hard slog of campaign work, persuasion, inspiration, inclusion and good old, bad old politicking, "Milk" says something about how progress is achieved in America. In that respect, it evokes the best aspirations of the country -- and, for that matter, of filmmaking.

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