Last month, an unknown, unemployed former soldier astonished America by becoming the Democratic nominee for South Carolina's upcoming senate election. How on earth did he do it?

The journey to the home of one of the most enigmatic figures to emerge into the American political scene involves a drive deep into rural South Carolina. The road, bullet straight and lined with tall firs and pines, continues for miles with only an occasional Jehovah's Witnesses church to break the monotony.

The entrance to Alvin Greene's house, which doubles as his political headquarters, is marked only by the street number stamped on its metal postal box. At the end of the driveway his brother, James Greene Jr, is cleaning a small collection of vintage cars in the blazing heat. He asks my business, then leads me through the garage to the back door which he knocks on loudly. Then he knocks again.

"What you doin' here so early?" comes a muffled voice from inside.

The door opens and James says: "There's someone to see you."

"Oh, yeah, OK. I said he could come," replies Alvin, who is wearing blue tracksuit bottoms and a grey T-shirt with an ARMY logo across the chest. Inside, the television is on, tuned to a news channel, in front of which an old man with frizzy white hair is sprawled under a blanket on the sofa. He looks asleep, or comatose.

Greene hurries me past and into a living room. I'm about to launch into questions, but he stands suddenly. "I've got to take a break," he says, then disappears into a back parlour.

A month ago nobody had heard of 32-year-old Alvin Greene. An unemployed army and air force veteran from Manning, South Carolina, he spends most of his time looking after the old man – his 81-year-old father, James Sr – and is not often seen outside the house.

At least that was true until 8 June, when the results were announced of the Democratic primary race to choose the party's candidate for election to one of South Carolina's two US Senate seats. To the astonishment of most who had followed the race, the party's preferred candidate, Vic Rawl, a retired judge who had run an aggressive campaign with a war chest of $250,000 and an army of volunteers, came a miserable second with just 42% of the vote. Trouncing him, with more than 100,000 votes and 59% of the total, was one Alvin Greene.

The South Carolina Democratic party was sent into a tailspin from which it is still recovering. Where did this Alvin Greene come from? He had never run for public office and had no experience of political campaigning. He doesn't own a computer and uses the one at the local library. He didn't have a website through which to marshal his troops. Come to think of it, he didn't have any troops. He had no mobile phone or donors, though he did print flyers. His name recognition among South Carolinians was close to zero.

So what happened?

Questions began to be asked, conspiracy theories cooked. The leading Democratic congressman in the state, Jim Clyburn, came up with the idea that Greene must have been planted into the race by the Republicans to destroy the Democrats' chances of winning the election proper in November. Others likened Greene to Forrest Gump and questioned his mental stability. To cap it all, it was discovered that he was facing criminal charges for allegedly having shown pornography to a female college student. Local and national newspapers had a field day. He was dubbed the "mystery man" and the "Manning-churian candidate". The banner headline in a local paper perfectly captured the mood: "Who the hell is Alvin Greene?"

An excellent question. And having come all this way to find the answer, it is a little distressing to see him vanish before the interview has begun. Several minutes later, he returns to the living room and slumps into a claret-coloured plastic sofa.

I begin by extending congratulations on his victory. How is he feeling?

"Good," he says, but no more comes.

I try a different tack, asking him what his life has been like since being thrown into the limelight.

"Busy." This time, happily, he goes on. " I didn't expect so much so soon, but I'm getting used to it."

It was a pretty big deal, I say.

"I was hoping that it would be big so I would be able to get my message out to as many folks as possible: getting South Carolina and America back to work, bringing South Carolina and America back. And it's not just America. We have got to get the world back to work and bring the world back. So it's a message that has to reach folks and it's something that I'm pursuing and it's going good."

It becomes clear fairly early in our conversation that Greene is not the greatest orator. He is not in the mould, say, of one of his heroes, Barack Obama. When I ask him whether Obama, as a fellow African-American, had been an inspiration behind his own decision to enter politics, he says, "Yes, I mean there's something that, you know, I knew so I just knew that. It was in my mind, I knew that, that, that, that the voters really, they really, that they really, erm, followed the candidate. That they really wanted substance in a candidate . . . "

It is clear, too, in the course of the two hours I spend with Greene that he has some pretty wacky ideas that, were he to win in November, would put him among the more unpredictable members of the senate. At one point, he lurches off on his big idea for how to create jobs in South Carolina.

"Another thing we can do for jobs is make toys of me, especially for the holidays. Little dolls. Me. Like maybe little action dolls. Me in an army uniform, air force uniform, and me in my suit. They can make toys of me and my vehicle, especially for the holidays and Christmas for the kids. That's something that would create jobs. So you see I think out of the box like that. It's not something a typical person would bring up. That's something that could happen, that makes sense. It's not a joke."

Similar statements made by Greene in the days since his victory have allowed the media, most recently on the Huffington Post yesterday, to present him in precisely that way – as a joke. Pundits have scrambled to explain how he pulled off his win. The voting machines must have been faulty; his name came before Rawl's on the ballot; the spelling of Greene is a black spelling that drew African-American voters; voters confused him with the singer Al Green.

The tone of some of the comments has bordered on cruel. The Democratic party elite tried to force him to stand down as duly elected nominee, but backed off when they realised they had no power to do so. Two Democratic politicians from the South Carolina assembly – both African-Americans – went to see him and tried to talk him out of his candidacy, and when he politely refused they suggested he was mentally impaired.

I ask him how the welter of conjecture and insinuation has affected him. He won the election handsomely, yet nobody seems able to accept it. "I just have to tune those things out. It's like people are in their own imaginary world and they want to hear what they want to hear."

Were the voting machines faulty? Is that why he won? "Nope. I got 60% of the vote, and 60% of the vote is decisive. That's more than Obama won this state with in the primary in his presidential bid. He got 55% of the vote. This is not a mistake. That's not a fluke."

What about the fact that he's facing charges for having shown obscene pictures to a university student? He hints that he has offered a deal whereby the prosecution would drop the charges in favour of a lesser non-custodial punishment. "First-time non-violent offenders should be entitled to pre-trial intervention programmes, like seminars or courses that would expunge the charges. I applied, I met the criteria. They could have dropped the charges."

And the suggestion that he is mentally ill? "That's an insult!" The answer is barked out, with distinct anger. But it doesn't come from Alvin. It comes from James Sr, who is shuffling past in his slippers just as we reach this point.

I turn to him and ask why he thinks such insults, as he sees them, are being levelled at his son. "Back in my day black people who registered to vote were turned away. They called the doctor and treated them as crazy."

Alvin's father, as a younger man, stood for office in the local council, but lost. I ask him whether he faced the same insults and opposition.

"Oh yeah."

What things?

"They told me I would never make it."

Then Alvin's brother, James Jr, the one with the vintage cars, joins in. "They are trying to scandalise my brother. They think he can win and they are trying to bring him down. They are making out he's slow. He's not slow. They say he's running for someone else. But he's running for himself. He's been talking to me about running for the past two years."

I'm starting to get the impression that the Alvin Greene-as-Forrest Gump narrative is falling wide of the mark. How about the line that he is a Republican plant? In that case, how to explain the certificate that he proudly points to on the wall for the politics degree he gained at university? Or the fact that he spent 13 years in the military, in both the army and air force, and though he was involuntarily discharged (no one yet knows why), it was honourably so. Or the tell-tale signs that he was brought up by his parents and teachers to respect public service – the line of plastic model presidents from Washington to Nixon that his late mother collected, lining the mantelpiece, and the photograph of an eight-year-old Alvin asking for the autograph of Charles Bolden, then a general in the military and now the administrator of Nasa. Conspiracy theorists have pointed to the $10,400 registration fee Greene had to pay to run, and questioned how he could afford it – to which he says simply he had saved up from his long military career.

"I followed politics as a child," he says. "I remember when Jesse Jackson ran for president when I was nine years old. He's a South Carolinian native. I made a campaign sign out of construction paper and put it out on the highway so folks could see it as they passed by."

He says he has wanted to run for election ever since he can remember, and that the idea got serious when he was posted to South Korea, from where he watched the implosion of the US and world economies and felt he needed to do something. "I thought I would start locally and work my way up," he says, "but it happened to work this way and I'm taking advantage of it."

Perhaps the ridicule that Greene has received from the TV networks and press is not entirely merited. That sense thickens when I head into town. Like many small towns in the south, Manning is dominated by a monument outside the central courthouse to the Confederate army and those who died in the cause of upholding slavery. Just off the central square Spencer Tindal is minding his barber's shop, shaving the heads of elderly black men. Tindal has known the Greene family for years, so how would he describe Alvin?

"He's a quiet person, but he's educated and he can do anything he wants to do. He has a politics degree."

And then Tindal reminds me that Greene is the first black person to be taken on as a candidate for a US Senate seat in South Carolina since the days of reconstruction immediately after the civil war almost one and a half centuries ago. "He's made history. And for black people like me of my generation, that's good. In our area, people like me don't have a chance to move up or make history. And he's doing it, he's trying to make a difference."

Back at Greene HQ I ask him whether he sees his election victory as a blow struck for South Carolina's black and under-represented voters. "Yes, that's part of it. Regular voters identify with me, the ordinary folks, the people who work every day, the everyday average typical person identifies with me more than any of my opponents, the leadership, the people behind the desk."

I leave feeling as baffled as when I arrived. Greene, South Carolina's Democratic candidate for the US senate is a mix of glaring inadequacy and raw political conviction. He repels and inspires, moments of lucidity interspersed with moments of the complete opposite. Next stop will be the main election in November in which he will face Jim DeMint, the Republican senator who will deploy all the firepower of an incumbent. Greene will be outgunned, outwitted and humiliated. He will lose in a landslide.

Now where have I heard that before?