I want to start with a question. Why do you think political parties exist? Because I think in the answer to that question lies the truth about just how profoundly the internet is going to change politics and journalism.

Here’s my own answer. Political parties exist because of the mass media. They grew up with the mass media. Before the mass media, we didn’t really have political parties. Sure, we had loose coalitions and factions, but the current monsters with their tight whipping and insistence that everybody say the same thing about everything? That came only after the newspapers and broadcasters had come first.

After the creation of the mass media, a very small number of people – proprietors, editors, political journalists – began to act as the gateway to the voter. So politicians organised themselves in tight groupings to make their message intelligible to that small group, to lobby them and to seduce them. There was little space for mavericks. They just confused things.

In his book The Long Tail, Chris Anderson described how limited shelf space for goods works. A shop can only fit a few things in its premises. The constraint is physical. For niche products, there wasn’t really a way of reaching customers at all.

The internet changes that, argues Anderson. A long tail of products with a small potential market (specialist books, for instance) can be easily obtained. And easily produced, too. New production technologies allow retailers to source, or even produce, single items when they are requested.

As it is with produce, so it is with politics.

This first struck me with force when the Labour MP Siôn Simon made that YouTube video impersonating David Cameron.

(“Want to sleep with my wife? That’s cool. Come down, check it out, we’ll sort it out. Safe. I’ve got two kids, kid one, kid two. You like them? Take one. That’s cool.”) It wasn’t so much that Siôn made an idiot of himself; it was, rather, what it cost to do that. In order to make that big an idiot of myself, I had, when working for the Conservative Party, required hundreds of thousands of pounds and M&C Saatchi. Siôn had required only a video camera and his fellow Labour MP Tom Watson. I realised then that the days when only a huge central operation could get any sort of message across were over.

To use another example, look at John Prescott. He has managed, using a BlackBerry and a blogging account, to communicate with voters without needing to have his message approved by anybody. And his success will draw in others. The result will be a weakening of central party control.

It is already happening in the Tory party.

The ConservativeHome website – a stunning endeavour, certainly one of the biggest developments in right-wing journalism for 20 years – is now an important rival to the official party site. It is the place where members meet, where campaigns start and where news can be exchanged.

I recently read a story on that site about the resignation of a Conservative Party area officer in Northern Ireland. The Conservatives have concluded an electoral pact with the Ulster Unionists. One consequence has been a dispute over the party’s name when the two fight together in Northern Ireland, and this caused the resignation. This is a tale too arcane to make a news story, but there it was on the site, with a frequently updated account of the whole row.

And ConservativeHome readers expect to find that sort of stuff there. The internet has allowed a relatively tiny number of party insiders and obsessives to find one another and form their own community. Without the party’s permission. Without the party’s database. Without the party’s money.

The internet has hugely reduced the entry cost for new publications. Iain Dale’s very popular blog, or the witty Guido Fawkes site, cost little more than the time of those who write them. With only themselves for backing, they are able to get very similar audience figures to (plucking a random example out of the air) my own Comment Central blog on the Times site. And the impact that the Guido site, in particular, has made on the political debate has been significant.

It will make it cheaper, too, for MPs to reach their own voters locally. The requirement for activists to deliver leaflets and knock on doors will diminish, as voters can be reached on their home computers. MPs may also gain some freedom from the national party.

So the internet will devolve power. But that’s not all it will do. It could be a powerful weapon in the hands of the party organisations, too. If they knew how to use it. So far, the parties have got to the point of developing better websites, making use of YouTube and trying a touch of viral marketing.

The Obama campaign of course did so much more than that. It was using the internet as a social networking tool. It was getting its activists to communicate with their friends over the net and using mobile phones. It harnessed their enthusiasm, getting them to make ads, ceding to them a tiny bit of control in exchange for a huge increase in reach.

The major parties have to catch up with what his campaign achieved, and quickly. The danger for them is not that their rivals in other parties will outperform them. It is that their own members and supporters will grab control, taking over the functions of party HQ without asking.

One more thing. The modern political party was the creation of the mass media. And the mass media are being replaced, becoming part of something much larger, sprawling, impossible to control.

It has hardly started.

Daniel Finkelstein of the Times runs Comment Central, one of Britain’s most popular political blogs