Midway through the presidential campaign in “Head of State,” favored candidate Brian Lewis tells his unconventional opponent Mays Gilliam (Chris Rock) that while Gilliam may be “entertaining,” he just doesn’t have what it takes to make it to the White House. In this accidental moment of perception, Lewis — whose campaign slogan is “God Bless America! And no place else!” — seems to have highlighted this film’s greatest flaw. Rock, despite his irreverent comic genius, simply cannot transfer his stand-up comic’s energy into success on the big screen.

Rock directs and stars as Gilliam, a Washington, D.C., alderman who starts off the movie having a very bad day. His fiancee dumps him, he gets evicted from his office, and his car is repossessed. But Gilliam’s life takes a cinematic turn when one of the presidential nominees is killed in a plane crash and a group of conniving politicians chooses him as the replacement, assuming that he will lose the election and help their own manipulative presidential pick win the minority vote in 2008.

Rock and co-screenwriter Ali LeRoi (“Down to Earth”) invoke a conventional Hollywood fish-out-of-water story very similar to the one in “Dave,” about a good-hearted everyman who finds himself in an unexpected position of power. The added twist is that in “Head of State,” the everyman is black. Though the film has amusing and provocative moments as it mocks racial stereotypes and the electoral process, ultimately it is neither as funny nor as powerful as it could have been.

The politics of the presidency are familiar territory for Rock, who covered the 1996 campaign for “Politically Incorrect” and who often satirizes the executive branch in his stand-up work. In fact, Rock appears most comfortable during the film when he returns to this style of commentary in the context of his onscreen political speeches. But other than racial questions, many of the issues raised in “Head of State” feel strangely dated in our current political climate. When the movie jokes about sex scandals, corruption in campaign finance and school shootings, for example, you feel as though you’re watching material left over from Rock’s Clinton administration comedy bits.

The movie censors itself at its only truly timely part, when all of the characters condemn Gilliam for a comment he makes about the U.S. policy of bombing places around the globe. This is one of many moments during the film when you wonder if Chris Rock would have avoided similarly controversial material during one of his HBO specials.

Rock’s finesse with racially-driven humor, his willingness to take risks and his courage to get angry are the hallmarks of his stand-up comedy, but they probably didn’t make the people over at DreamWorks too happy. Elements like a cheesy ending and an unnecessary romantic subplot sweeten Rock’s bitter medicine with a generous helping of Hollywood sugar. With his edge diluted, Rock has only a weak plot and an awkwardly-paced, overly-sentimental script to entertain us. Unfortunately, it just isn’t enough.

“Head of State” is most effective in the rare moments when it dares to be tougher. Like when a white man jumps out from the crowd during one of Gilliam’s speeches and yells “Get your hand out of my pocket!” in an allusion to the assassination of Malcolm X. Or during a fast-forwarded and slickly-edited montage that juxtaposes images of white people rushing to the polls to keep Gilliam out of office with footage of the inaugurations of Nixon and Reagan. My favorite part is when Chris Rock tells an elderly white woman who wants a ride on his pimped-out campaign van to “Get to the back of the bus!” But these more potent segments get lost in what is predominantly just a messy comedy.

The movie ends up analogous to the political campaign Gilliam wages: Despite its controversial black headliner and attempts to deal with real issues, it ultimately submits to the formula of the established system.