Good guys never get elected to office, they're always appointed when the genuine article gets caught with his pants down, his hand in the till or his butt in a sling. At least that's what Hollywood would have us believe, which is not to say it isn't so.

In every case, political scoundrels try to manipulate these greenhorns, only to discover that men of the people cannot be outwitted or compromised. Meanwhile, the gullible heroes are shocked to find that the government has been corrupted by cynics, crooks and pork-barrelers. Then they actually manage to fix the system.

This idealistic hokum harks back to Frank Capra's 1939 classic, "Mr. Smith Goes to Washington," and most recently resurfaced in the holier-than-thou TV series "Mr. Sterling." Today, Chris Rock joins the party in "Head of State," a slight populist fantasy that the star also co-wrote and directed.

Rock plays Mays Gilliam, a decent D.C. alderman who represents Ward 9, a neighborhood "where you can get shot getting shot." Mays is recruited to run for president in 2004 after the Democratic candidate and his running mate are killed in a plane crash.

Mays's opponent is the sitting vice president (Nick Searcy), an oily Southern baby-kisser with a commanding lead in the polls. Not that it matters -- Sen. Bill Arnot (James Rebhorn), the Democratic Party chairman, picked Mays because he can't possibly win. Somehow or other this is supposed to lead to Arnot's candidacy and subsequent victory in the next presidential election.

To make sure Mays loses, the senator assigns two trusted staff members (Lynn Whitfield and Dylan Baker) to mismanage his campaign. At first, Mays plays the patsy. He okays sappy bucolic campaign ads, wears tasteful suits and delivers the bland speeches provided by his team.

When Mitch (ebullient Bernie Mac), Mays's brother and future running mate, attends one of his blah speeches, both the campaign and the movie pick up much-needed energy. Mitch urges Mays to be himself, take off that suit and start telling it like it is. Street talk meets straight talk, and suddenly there's a jump in the polls.

Like Queen Latifah in "Bringing Down the House," Rock taps into the tired and bogus conceit that white folks can't jump or jive, so he helps them get their groove on. So many movies have addressed the problem already that it's hard to believe that there are any rhythmically impaired whites remaining in the contiguous United States.

Though the comedy falls short of a debacle -- which is what such egocentric projects tend to be -- it isn't as sharp, fast or funny as Rock's stand-up routines. He's invariably steamrolled by the bigger-than-life Mac. He's even shown up by Searcy, whose smarmy veep can hardly hide his contempt for the proles. His slogan, "God bless America . . . and no one else," is certainly in keeping with the times.

Head of State (90 minutes, at area theaters) is rated PG-13 for language, sexuality and drug references.

Chris Rock, left, is a D.C. alderman recruited to run a losing campaign for president in 2004 and Bernie Mac, right, is his running mate.