Over the past 15 years, Bragg has done more than any other to shape a British political consciousness through music - he formed Red Wedge in the Eighties, the glorious but doomed attempt to back the Labour Party.
Bragg was near tears at an election-night gig when Old Labour suffered its last defeat, and must have been tempted to weep since the New Labour victory last year. "The labour movement remains a mass grassroots movement," he explains earnestly. "That's the real broad church, not the Labour Party. And there are some New Labour MPs who are actually still in the labour movement".
Even so, in this post-ideological age, what is left of the Left? Bragg defines the new politics as a "socialism of the heart": "You have to ask yourself, in a time when socialism seems to have lost a lot of its meaning, what do we actually believe in, instead of a word or an ideology? 'Socialism of the heart' seemed to be a way to make people not think that we were just giving up and going away and looking for soft words that mean nothing. Compassion does mean something; empathy does mean something. They're the roots of a caring society."
It would be easy to see his pronouncements as those of an earnest agit- prop politico who never quite recovered from the Conservatives' slash- and-burn attitude to industrial relations and social cohesion. But Bragg is anything but a self-righteous ideologue. He's a truly engaging mix of political sophistication and gentle bloke-ish charm. Bragg shakes hands like he means it and has made no attempt over the years to change that Arfur Mullard diction or his personal style (very casual, DMs). He's still the thinking woman's socialist symbol.
This 40-year-old godfather of political pop has seen the days of rage turn into the days of rave. Chumbawamba redefined its much-vaunted anarchism and took the major-label coin, Tubthumped its way into the charts via Top of the Pops and threw a bucket of water over John Prescott, one of the few working-class members of the Cabinet. The Manic Street Preachers topped the pop charts with a song that declared, "If I can shoot rabbits then I can shoot fascists." Is this a sign of political guerrilla action in pop or simply catchy tunes capturing a huge audience?
"Those kind of bands always give me encouragement but what you've got to say is, 'what are you doing about this other than just singing a song and being on Top of the Pops'?
"What I want to see is some action. How many people do you think who bought the Manics' single actually care about shooting fascists? But you have to trust people to make their own connections. You can't ram it down their throats and you can't follow them home after the gig to make sure that they'll live their lives the way you have written the song."
Bragg has often been typecast as the working-class hero but in one of his best-known songs, "A New England", he declared: "I don't want to change the world, I'm not looking for a new England." He resists the persona of Marxist messiah. "People in the music business often like to encourage the idea that the pop singer is some kind of social shaman," he says, "who will focus on society's ills and do something about them with a wave of his magic wand from the back seat of his white Rolls Royce. That's patently absolute bollocks," he exclaims.
"I get letters saying, 'because of your records, I became a union lawyer'. But I didn't take the exam, I didn't pay for him to go to college. I'm not standing in the courtroom representing the union. He is. If I was the soundtrack to him achieving that, then I'm very pleased. But I can't sit here making records thinking, 'are people going to be inspired to live pure, clean lives by listening to this?'"
It is easy to see how Bragg's vision and political commitment made Nora Guthrie believe that he would be the natural inheritor of her father, Woody's, legacy. Woody was the original singer-songwriter, inspiring Dylan, Bragg, Springsteen and 1,000 imitators who don't even know his name. He died of Huntington's disease in 1967. Nora set Bragg loose in her father's archives and the result was a stunning rediscovery of his work.
Bragg and US country-rockers, Wilco, set Guthrie's lyrics to music and released the CD, Mermaid Avenue, earlier this year. "I've learned so much about myself from looking at Woody. The experiences I had in the Eighties and Woody had in the Thirties were quite different but the conclusions that we came to were very similar."
The BBC filmed the recording of Mermaid Avenue for a documentary to be shown later this year. Woody Guthrie was a left-wing Renaissance man, Marine, broadcaster, writer, father, husband, womaniser, drinker, icon and iconoclast. It's almost a description of Billy Bragg, past and present.
The hard-living days are long gone, transformed by his partner Juliet, their five-year-old son, Jack, and Juliet's son, Jamie. Juliet Wills was a director of Bragg's former record company, Go! Discs, and theirs is a relationship which has evolved through work, into friendship and, ultimately, love and parenthood.
Bragg did not follow Woody into the Marines but spent three months in the army when he was 23, bereft of personal, political and musical direction. He left after an unlikely sabbatical which had given him breathing-space, the ability to drive a tank and an unhealthy interest in Nolan Sisters album covers.
Sitting opposite me now, Steven William Bragg has come a long way: from his birthplace in Barking in fact, to hanging out with heads of state and rock mega-stars. He's in a reflective mood today, partly because of the publication of his official biography, written by Andrew Collins, and is partly because he's re-assessing his personal New World Order.
"Being 40, having been in the industry for 15 years, and representing a line-back to punk, you can write the story of the Eighties as a backdrop to my own personal story. It's about how you get from being working class, white, poor and living in your mum's back room playing guitar, to standing on stage at Wembley stadium with Nelson Mandela."
In his twenties, Bragg had three career ambitions: make an album, tour America, be on the cover of NME. He's achieved all three. "When I made that wish-list, I was on a trajectory to escape the fate that life had offered up to me because of where I was born and how I was educated. I was trying to escape my circumstances. I'm not trying to escape my circumstances any more. But I am trying to get a balance between my career and my family life."
It's refreshing to hear a man talk about his family as Bragg does; you just know he'd never use the word "baby-sit" when talking about staying in on his own with the children. "I wondered what 'Billy Bragg, someone's dad' would sound like. I wrote a song, "Brickbat", about my new domestic situation. It genuinely moved me because I realised I could write songs in this new life situation, and that I still had something to articulate. As your audience gets older, they do a lot of things in their lives and you can say, 'I'm not going to do that, I'm going to stay young, I'm going to wear dark glasses and comb my hair in a quiff, I'm going to wear leather trousers and a spandex shirt'. Get a life! There are a lot of pop stars who would benefit from getting a life."
Bragg says on "Brickbat": "I used to want to plant bombs at the Last Night of the Proms/Now you'll find me in the bathroom with the baby." He did go to the Proms this year, not to plant bombs but for a recital of political music. The work of composer and anti-fascist, Hans Eisler, who fled the Nazis only to be hounded in the McCarthyite witch-hunts of Fifties America, was featured this year. Woody Guthrie's song about Eisler's persecution is on Mermaid Avenue.
In the manuscript of one of his compositions , Eisler had written: "In a society that understands and comes to love a songbook like this, people will live well and in safety. It is in the hope of achieving such a society that these pieces have been written." It could be Billy Bragg speaking.
It's a tribute to Bragg that he hasn't suffered political burn-out and compassion fatigue. He's got a lot of Tubthumping years left and he's looking forward to the next stage of his life. "I'm very comfortable about being 40. Where I used to get by on energy, I now have experience and the insight that comes with that," he says. "I read something that [New Order's] Bernard Sumner said, that he wrote political songs, but it was about time that someone else did. I feel totally like that myself. Why should I always have to be the poor bastard who's writing the political songs?
"I don't mind doing it and I'm very happy to turn out for the Dockers and all that. But I'm also ready for the next generation to come along and make their voice heard, and make their punk and have their moment. We do ourselves a disservice by looking for white guys with guitars all the time".
But some white guys with guitars really are worth looking for.
Billy Bragg plays The Forum, London, on October 20. Live on-line interview for AOL subscribers, 8-9pm on 21 October. 'Still Suitable For Miners. Billy Bragg: The Official Biography', by Andrew Collins, Virgin, pounds 12.99
PLENTY TO BRAGG ABOUT
On what's left of the Left
"Compassion does mean something; empathy does mean something. They're the caring roots of a caring society"
On the reality of political pop:
"You have to trust people to make their own connections. You can't follow them home after the gig to make sure they'll live their lives the way you have written the song"
On his life story:
"It's about how you get from being working class, poor and living in your mum's back room playing guitar, to standing on stage at Wembley with Nelson Mandela"
On being 40:
"Where I used to get by on energy, I now have experience and the insight that comes with that"
On his ambitions:
"I am trying to get a balance between my career and my family life"
On being the political singing voice:
"I don't mind doing it. But I'm also ready for the next generation to come along and make their voice heard and make their punk and have their moment"