Hanging on a wall in Doug Stanhope's Arizona home is a framed letter, written in 1979 by his school psychiatrist. Concerned about the then 12-year-old Stanhope's obsession with sketching bloody limbs and smutty cartoons, it addresses his mother with the opening gambit: "First of all, we strongly believe that Douglas is in need of professional help …"

Over three decades later, any random observer at one of Doug's stand-up shows might think said psychiatrist had a point. Almost always drunk, Stanhope's topics include such crowd-pleasers as questioning whether child abuse is really as bad as victims say it is, claiming he assisted his own mother's suicide, and promoting genetic mutation on the grounds that it might make sex more interesting. He has a book out – if you dare search for it – called Fun With Pedophiles: The Best Of Baiting. Michael McIntyre, he is not.

"I go onstage, it's like I'm leading you into battle," Stanhope sometimes tells his audiences. "You're not all going to be here at the end."

Based on all this, a certain sense of apprehension is perhaps the natural response to meeting Stanhope. It doesn't help that, when conducting a Google search of recent interviews, one recent piece that came up used the following Stanhope quote as a headline: "Guardian people? Fuck them …"

Yet here we are, drinking afternoon double vodka and sodas in the Pimlico apartment he never leaves (he hates London, as he does most big cities) and Stanhope seems kind of sweet, and kind of sensitive.

'I immediately split the crowd. I thought about coming on every night and shouting, "Gay pride, white power!" just to confuse people'

Photograph: Sarah Lee for the Guardian

"I've had some bad shows where I just sucked," he says of his recent run at Leicester Square Theatre, "but I've had some assholes, too. Some guy stood up Saturday night and said 'This is the same shit you've been peddling the last five times you've been here.' That's your biggest fear: someone who knows every word you've ever said."

How did it end? "Oh, with him being escorted out."

Stanhope doesn't laugh at this. Instead he looks hurt, like a big old teddy bear that needs a hug. He continues: "You know, I'm amazed I have this much of a fanbase. I immediately split the crowd. I thought about coming on every night and shouting, 'Gay pride, white power!' just to confuse people."

He's sounding morose but suddenly someone walks past and Stanhope kicks into life: "Hey man! How are you doing? Good!" He turns back around, smiling. "Bartender," he explains. "We've come to know each other ..."

At school, Stanhope says he was too dark to be considered the class clown and, after a spell as a "fraud telemarketer" ("borderline legal stuff, trying to scam people basically"), he decided to give stand-up comedy a go at an open-mic in Las Vegas. "I was 23 with a mullet doing lots of jerk-off material," he says. "I can't look at the old tapes now." Maybe not, but within six months he was working as a house MC in Phoenix. By the early noughties he was the name on every edgy comedian's tip list, and gained a further cult following through his cameos on Charlie Brooker's Newswipe, although any Guardian readers who liked his spots on there might still be advised to approach with caution. When I put it to him that, like Brooker, there's a liberal heart beating behind the misanthropic exterior (he's fiercely pro-choice and pro-drugs), he disagrees. Later that night, onstage at Leicester Square Theatre, he'll go on a dubious rightwing rant before ending with, 'Fuck you, Guardian, for calling me a liberal.'"

So what are Stanhope's politics? He maintains he's never been all that political, but can that really be true? One of the thrilling things about Stanhope's material is that, when it really works, it offers a refreshingly honest take on life, often exposing our own double standards. Even when he talks about anal porn he spins it into a message about how they should teach it in schools as a way of controlling Earth's rapidly rising population. These are often inherently political points he's making. Besides, if Stanhope really didn't care for politics why did he run for president in 2008?

"It seemed like a funny thing to do!" he says. "I thought we could maybe get on the ticket of the Libertarian Party. But people were either amused or horrified at the idea of me representing their party."

How come? "It made me realise the sheer breadth of all the shit I don't know about politics. I thought I'd study, so I read an actual book called Politics For Dummies or something but I just couldn't retain it all, it was like school."

Hang on, so he was basically cramming to be president?

"Yeah, I was; I was really trying and that sucked all the fun out of it. It was the worst period of my comedy career. My friends didn't want to be around me. It was the best decision I made when I said, 'Fuck it, I'm not doing this.'"

'I'm strongly debating quitting. I don't want to create things to be angry about, I'd sooner start doing happy shit'

Photograph: Sarah Lee for the Guardian

After pulling out, Stanhope switched his allegiances to Ron Paul – "I look at it like I look at football teams, it's a bit of fun so I root for the underdog" – then switched again to Barack Obama once Paul was knocked out of the race. Consistency isn't his strong point, but then pinning him down is somewhat pointless. The more we talk, and the more you listen to his old material, the more he seems less like the righteous Bill Hicks type "lazy" journalists like to compare him to, and more a Charles Bukowski-esque character: a drunken deadbeat throwing out tales from America's seedy underbelly without caring too much what the "message" is. Sometimes they just so happen to make a mockery of the absurd rules and rituals human beings live their lives by, at other times they're just jokes about blowjobs. His live show is as unpredictable as his position on the political spectrum; in the last year alone the Guardian has watched Stanhope pull off a sensational show, and also seen him aim half-formed rants at all the wrong targets (drug addicts, prostitutes) without locating anything close to a punchline. Given the subject matter, does he ever feel a sense of responsibility with his act?

"Yeah, well …" The cigarette that's been twitching in his hand during the interview finally needs to be smoked. Standing on the pavement outside he opens up, discussing a fan letter he was sent.

"It was from this 23-year-old kid, and it was all fanboy for the first paragraph. Second paragraph, it said, 'By the time you read this I'll be dead.' We looked into it: he was facing seven years for looking at child pornography. We fact-checked, it was all legit …" he trails off. "So I don't feel responsible but it does affect me. Because I do have a lot of fucked-up people coming to my shows, and people who will travel a long fucking way. So you want to do the bits they like or whatever …"

He trails off again. Often during a routine he will joke about his heart not being in stand-up any more and maybe this is why. He might claim not to give a fuck, but you sense that he constantly feels like he's letting people down. Some have suggested he's depressed, but he actually seems happier than ever in his domestic life (in true Bukowski style, Stanhope got together with his partner of six years after inviting her to take hallucinogens with him at a DIY Burning Man party he was having with friends in Death Valley).

"I've got a great relationship, a nice house, great dogs," he sighs. "We're strongly debating quitting. I don't want to create things to be angry about, I'd sooner start doing happy shit …"

Doug Stanhope, then: the comic who's pissed off because he's happy. The guy who can turn anal sex jokes into political statements, yet run for president because it's a laugh. And now, perhaps the ultimate contradiction: the bile-filled angry comic who's actually … softening?

"Yeah maybe," he says, smiling, "and if people don't like it they can fuck off!"