There’s a singular moment in nearly every Will Ferrell comedy where the movie falls apart.

It’s not necessarily the humor that wanes, it’s the overall movie: the structure, the theme, the believability.

“The Campaign” — about an increasingly malicious congressional race in North Carolina — holds together longer than Ferrell’s other broad efforts, such as “Old School,” “Anchorman” or “Semi-Pro.”

Could be because the sharp screenplay (by Chris Henchy and Shawn Harwell) isn’t as susceptible to Ferrell’s meandering improvisation. Or because he splits his screen time with Zach Galifianakis, who delivers a quirky yet heartfelt performance that actually exudes some humanity.

Ferrell plays Cam Brady, a long-serving congressman who runs on a platform of “America, Jesus, Freedom.” Deep down, Cam is a womanizing phony whose only real skill is getting re-elected. So far that hasn’t been a problem, until the billionaire Motch brothers (John Lithgow and Dan Aykroyd) decide they need a new political lackey to help approve legislation that will “insource” American jobs to transplanted Chinese sweatshop workers.

The Motches (a parody of Wichita’s Koch brothers) select novice Marty Huggins (Galifianakis), the black-sheep son of a political ally. Clad in mom jeans, professor turtlenecks and old-man sweaters, Marty seems unelectable. His own father tells him, “You look like Richard Simmons crapped out a hobbit.”

Initial polling shows Marty maintains a mere 26 percent “likability factor” with voters, and the top three phrases they use to describe him are “odd,” “clammy” and “probably Serbian.” But the pudgy, mustachioed Marty has something the slick Cam doesn’t: integrity … sort of.

“The Campaign” delivers some outrageous sight gags, ranging from a police dashboard camera view of Cam’s failed drunk test to the graphic fallout from the candidates scuffling to kiss a baby.

Even better is the film’s knack for introducing colorful supporting characters. This isn’t just the Will and Zach show, but a real ensemble piece that liberally portions out laughs to the lead characters’ wives, siblings and co-workers. (KC’s Jason Sudeikis earns plenty of screen time as Cam’s sensible campaign manager.) The highlight features a dinner table conversation at Marty’s house where he encourages his relatives to unburden themselves of secrets.

Director Jay Roach mined similar uncomfortable laughs in this year’s Emmy-nominated McCain/Palin biopic “Game Change” on HBO. Stylistically, “The Campaign” falls more in line with Roach’s “Austin Powers” franchise — fast-and-loose comedy that succeeds better here than the “Powers” sequels.

Yet the film loses potency as the candidates’ dirty tricks get more and more ridiculous. When is that specific moment the movie falls apart? It’s right around the time Cam seduces Marty’s frumpy wife (well-played by Sarah Baker) as fodder for a campaign commercial. A desperation to the proceedings sets in, a cartoonishness. And the plot gets uncharacteristically preachy to compensate.

The talented but manic Ferrell struggles to dial down his persona in this type of comedy. He resembles the political talk show hosts who are always shouting, as if that gives their words more meaning.

If the star could have found a way to match Galifianakis in subtlety, “The Campaign” might have significantly upped its likability factor.

What others are saying

• Michael Rechtshaffen, the Hollywood Reporter: “Will Ferrell and Zach Galifianakis are amusingly on point as a pair of mud-slinging contenders for Congress, the platform is a wobbly political satire that flip-flops chaotically between clever and crass, never finding a sturdy comedic footing.”

•  Karina Longworth, the Village Voice: “Plopped into a decidedly grey election season landscape infected with corporate rot on all sides, the film’s veer into fantastic solutions to very real and relevant problems is almost charming — but like a third-party candidate, it’s also incredibly easy to ignore.”

•  James Rocchi, BoxOffice.com: “ ‘The Campaign’ depicts both Democrats and Republicans as greedy, and goes further to say that greed and power make ‘sides’ irrelevant. If the film has any position, it’s that huge amounts of anonymous money given by shadowy manipulators to any, either, or both parties are bad for democracy. The great thing about ‘The Campaign,’ is that it’s weird and strange and surreal and passionate and dirty, a crazy-smart comedy where you laugh until it hurts — in no small part because a lot of it should.”