Politics makes for strange costars — or at least strange costar pairings. Will Ferrell and Zach Galifianakis aren’t quite Mutt and Jeff, but they’re close. Ferrell, tall and narrow and clean shaven, sits across a table from Galifianakis, not so tall and not so narrow and definitely not clean shaven.

They’re in a private dining room at Bistro du Midi, by the Public Garden, to talk about their new comedy, “The Campaign.” It opens Friday. They’re also there to recover from their first encounter with a duck boat ride. Later that afternoon they’ll pay a visit to the USS Constitution.

“Being on this tour,” Galifianakis says, “it feels like we’re on a political campaign. I don’t see how the candidates do it. . . . Comedians, you really only have to do it on the stage. When Will and I are walking around, we’re not necessarily wanting that attention. We’re just hanging out. But politicians have to do it 24 hours a day.”

JOSH REYNOLDS FOR THE BOSTON GLOBE

Will Ferrell (left) and Zach Galifianakis recently taking a duck boat ride through Boston to promote their new movie, “The Campaign.”

Ferrell’s political background is comedic: his dead-on “Saturday Night Live” imitations of President George W. Bush. Galifianakis’s is more direct. As Ferrell points out, he has “a political lineage.” His uncle Nick was a US representative from North Carolina, a Democrat, who lost a 1972 Senate race to Jesse Helms.

In “The Campaign,” it’s Ferrell who plays a Democratic congressman from North Carolina, Cam Brady. Brady thinks he’s running unopposed – until Galifianakis’s Marty Huggins, a local ne’er-do-well who’s as amiable as he is feckless, gets tabbed to run as Brady’s Republican opponent. All electoral hell breaks loose, and negative campaigning never had it so good.

How out of hand do things get? In a scene that’s already become an Internet favorite, Ferrell’s character inadvertently punches a baby. Later, he punches a dog. So which heinous act did the actor enjoy more?

Ferrell handles the question with a politician’s aplomb. “Comedically, it’s more fun to punch a baby, because it’s so horrific when you actually think about it — and unexpected. The audience just doesn’t think we’re going that far.”

Now he switches gears, the comedian taking over. “Plus, I have to say, that little baby really deserved it. So many demands. Such a diva. Did a great job, but,” his voice trails off as he shrugs his shoulders. “I was a hero to the crew.”

Asked if he felt envious over not having the opportunity, Galifianakis smiles even more sweetly than usual. “I’m a pacificist, especially with babies and dogs.”

The big problem facing a political comedy, especially a political comedy these days, is how to be outrageous about a process that has itself become so outrageous.

JOSH REYNOLDS FOR THE BOSTON GLOBE

Ferrell (left) and Galifianakis (right) posed with Jamin' 94.5 contest winners, from left, Ryan Shea, and Phil Morris .

Ferrell recalls watching the Republican presidential debates during the filming. “Our first reaction was, ‘Isn’t this fantastic! It’s exactly what we’re doing with the movie.’ Then we stopped and looked at each other and said, ‘Ooh, are we going far enough? It’s getting so nutty.’

“But I think we both feel like we struck that balance. It’s still pretty outlandish, the things these guys do [in the movie], and at the same time when the movie’s over people kind of stop and say, ‘Ooh, we’re not that far off.’ ”

Galifianakis nods. “Sadly, it’s more of a mirror than it is anything else.”

“The Campaign” has an impressive cast. Dan Aykroyd and John Lithgow play the right-wing Motch brothers, who bankroll the Republican candidate. Jason Sudeikis is Ferrell’s long-suffering campaign manager. Dylan McDermott is his Republican counterpart. Brian Cox, the original Hannibal Lector and most recently seen in Ralph Fiennes’s “Coriolanus, plays Galifianakis’s father.

JOSH REYNOLDS FOR THE BOSTON GLOBE

Ferrell and Galifianakis on the duck boat ride through Boston.

Not all of those actors are primarily associated with comedy. Does that change the equation at all for performers who are, like Ferrell and Galifianakis?

“It’s not necessarily different,” Galifianakis says, “though you’re just a little intimidated if you’ve seen Brian Cox’s work and then you’re in a scene with him face to face. In the back of your mind is, ‘One of us doesn’t belong here — and it’s me!’ ”

Where some comic actors specialize in situation comedy, Ferrell has cornered the market on vocational comedy. He’s starred in movies about stock-car racers, cops, TV newscasters, basketball players, figure skaters, and, of course, Christmas elves. Now that he’s played a politician, what profession is next?

There’s a long pause. Then inspiration strikes.

“Well, after today, duck boat captain. That’s my next movie, duck boat captain.”

Galifianakis now takes over the role of interviewer.

“Will that be a drama?”

“Yeah, I think it’ll be a drama. In the tone of . . .”

“ ‘Terms of Endearment’? Something kind of serious and heavy, around a sickness in the family?”

“Yes!”

“But a lot of the dramatic scenes are on the duck boat, where people go ‘quack, ‘quack.’ ”

“It’ll be kind of like ‘Das Boot.’ ”

“ ‘Das Duck Boat’?”

“ ‘Das Duck Boat,’ yeah, all shot in the interior of the duck boat.”

Ferrell looks at Galifianakis. Galifianakis looks at Ferrell. Both seem rather pleased. The campaign is definitely over. Consensus rules.