The internet is a levelling force. It diffuses power and empowers new competitors to challenge old arrangements. Elite newspapers and magazines, for instance, dominate their markets partly because it costs so much to build conventional hard-copy competitors. But the web has also allowed tens of thousands of new voices to find audiences at little cost.

Some of the same effect is already evident in US politics. Once it took years of heavy spending on direct mail and other recruitment methods to build a national membership organisation., the online liberal advocacy group, acquired half a million names with virtually no investment just months after posting an internet petition opposing former US president Bill Clinton's impeachment in 1998. MoveOn, and groups like it on the left and right, chisel away at the power of the main political parties by providing an alternative source of campaign funds and volunteers. But otherwise, the two parties that have defined American political life since the 1850s have been largely immune from the centrifugal current of the internet era.

Joe Trippi, a principal architect of Howard Dean's breakthrough internet strategy in the 2004 Democratic presidential campaign, is one of many analysts who believe that may soon change. The internet, he says, could ignite a serious third party presidential bid in 2008. "This is a very disruptive technology," he says.

The internet could allow an independent candidate to identify more easily an audience and financial base, just as it has allowed blogs such as the liberal Daily Kos or conservative Instapundit to find like-minded readers. More precisely, the internet has allowed readers to find those blogs. And because the audience mostly finds the product, rather than the other way around, the cost of entering the market is radically reduced. Mr Trippi believes an independent presidential candidate could organise support through the internet just as inexpensively. If he struck a chord, such a candidate could raise as much as $200m over the internet "in the blink of an eye", Mr Trippi predicts. It might not be quite that simple. But the two parties are pursuing strategies that create an opening in the centre of the electorate, even as the internet makes it easier for new competitors to fill it.

Influenced partly by Ross Perot's strong showing in the 1992 presidential race, Mr Clinton argued that capturing the middle was the key to electoral success. After an initial lurch left, he doggedly pursued centrist voters by breaking from liberal orthodoxy on welfare, trade and other issues. By contrast, George W. Bush, the US president, has been more willing to risk alienating moderate and independent voters to advance ideas that energise his base. He won re-election largely by increasing turnout among Republicans and conservatives.

More and more Democrats see their future in Mr Bush's model, not Mr Clinton's. Mr Trippi argues that Democrats are more likely to win back the White House by increasing turnout among their own supporters with a pointedly partisan message, as did Mr Bush. Not only have liberals such as Mr Trippi drawn this conclusion; so too have some centrists such as Simon Rosenberg, founder of the New Democrat Network. Mr Bush's success in 2004, says Mr Rosenberg, has rejected "for all time" the idea that the only way to win the White House is to win the centre.

This argument among Democrats is far from settled. But a tilt in Mr Trippi's direction is evident in the surprisingly unified Democratic congressional opposition to Mr Bush's priorities. The result is that both parties today are offering policies and messages aimed primarily at their core supporters. Even strategists such as Mr Trippi, however, acknowledge that by ceding the centre, both parties may be vulnerable to a new force.

The hurdles for an independent presidential candidate remain formidable. Even one with a competitive share of the popular vote may have trouble with the US electoral college vote system.

Yet if the two parties continue on their trajectory, the backdrop for the 2008 election could be massive federal budget deficits, gridlock on problems such as healthcare costs, fights over ethics and poisonous clashes over social issues and Supreme Court appointments. In such an environment, imagine the options for John McCain, the Republican senator from Arizona, if he does not win the 2008 Republican nomination, and former Democratic senator Bob Kerrey of Nebraska, now that he has dropped the idea of running for mayor of New York. If the two Vietnam veterans joined in an independent ticket, they might inspire a gold rush of online support, making the two national parties the latest example of the internet's ability to threaten seemingly impregnable institutions.

The writer is a columnist for the Los Angeles Times

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