In his father's footsteps. Straw the younger's political trek

Now a noted blogger, the mature Will tells Matthew Bell he aims to stay true to his ideals

If a knack for reinvention is vital in politics, Will Straw has a long and brilliant career ahead of him.

Twelve years ago the then 17-year-old son of the Home Secretary, Jack, made an unplanned debut in the media by being caught selling cannabis to an undercover Mirror journalist, who paid £2,000 to one of his friends for a meeting. Such a cruel introduction to the public eye would put many off, but not Straw. "It made me more careful," he says now, "but it was gutter journalism; there is a clear distinction between that and proper journalism."

Within a couple of years, he was back in the press, embarrassing his father by campaigning for free higher education while a student at Oxford. Now, seven years later, Straw is making a name for himself yet again, this time as a leading left-wing blogger. After a two-year stint in America last summer, he returned home to raise some money and launch Left Foot Forward, an "evidence-based" blog for progressives. It has been a meteoric success; monthly unique users have hit 20,000 and last week the blog was placed 17th in the Wikio rankings of political blogs – the 4th highest of the left-wing sites.

When we meet in a south London pub, not far from the scene of the canabis sting, there is no hint of the grungy past: immaculate in a grey suit and blue shirt, his hair close-cropped, Will Straw is the model young, urban professional – all BlackBerries and cappuccinos (he doesn't allow himself a drink until after 6pm). Perhaps he's too slick, the political jargon coming thick and fast. But Left Foot Forward is undoubtedly an influential and serious operation. To fund it Straw approached a number of philanthropic Labour sympathisers such as the EU's new foreign minister, Baroness Ashton, her husband, Peter Kellner, and Henry Tinsley, the former chairman of chocolate-maker Green and Blacks. They each donated four-figure sums, enough to employ him and another blogger, Shamik Das, full time.

The site's aims are clear. Inspired by the US site Think Progress, it campaigns for progressive policies on the economy, green issues and social reform, which Straw believes can breathe new life into Labour. The site has a three-point plan: to relay progressive policy ideas; to respond to Tory ideas; and to highlight examples of media manipulation, "especially from the Mail and the Express."

He talks with the steady fluidity of a trained politician. Take this response to being asked whether he has mellowed since his Oxford days: "I like to think I'm a little more grown up. We all go through life holding particular points of view dearly. Sometimes you are proved right and sometimes you are proved wrong. You would be an idiot not to learn from the times you are proved wrong."

The new, mature Straw reveals that he hopes to become an MP. If not surprising, it's a decision he has taken some time reaching. In 2003, after spending a year as president of the student union at Oxford, Straw joined the Treasury and worked there for four years, while the economy was booming. "Some people were saying 'we're storing trouble up for ourselves in the housing market' but I can't pretend I could see the crash coming," he says.

After winning a prestigious Fulbright Scholarship to Columbia University, he stayed on in Washington to work for a think-tank whose policies left a strong impression on him. He has ideas for social reform, the economy, taxation and foreign policy which he would like the Labour Party to adopt. "Doesn't everybody in politics hope that they might write the party's manifesto?" Although he is careful not to say it, he hints that Labour needs a new direction. "I've never been too keen on the distinction between New and old Labour. It was basically a branding exercise – an attempt by Blair to make the Labour party a viable government."

Straw is no rebel, but will criticise the party when he believes it has failed, such as its over-reliance on targets, and its tardy response to renewable energy. Inevitably, he is loyal to his father, although he says they frequently disagree on policy. "I probably am left of my father." But is he disappointed at his father's move to the centre since his Communist youth? "When you are young, you see things in black and white, but as you get older the shades of grey get clearer," he says, "The most important thing is not to forget what got you into politics in the first place."

In his case, it was playing football in Kennington with children from council estates and realising the injustices engendered by class and upbringing. But has he also abandoned his youthful left-wing views? "I think we are entering a more nuanced period in our response to capitalism. I hope I am true to my younger self." Let's ask him again in 12 years' time.