By Kevin Connolly
BBC News, Washington
Ralph Nader's announcement that he is running for president again in 2008 has provided us with perhaps the least surprising surprise of the campaign so far.
Ralph Nader's role could be pivotal in a tight race
Mr Nader has played some sort of role in every race for the White House since 1992 although his record in public life stretches back further still.
As a writer and political activist, Mr Nader has been an influential figure in American public life for 40 years - the scourge of corporate greed and wasteful defence spending and champion of consumer rights and the need to protect the environment.
His public life as measured through the ballot box is not perhaps very successful - he achieved 2.74% of the popular vote in 2000 for example - but his supporters argue that he is a kind of grassroots visionary whose ideas start life on the fringes of political debate then move to the centre.
Consider how all serious politicians are now expected to talk about the environment, they say.
Even if he does not affect the outcome, the candidate who now replaces John McCain as the oldest man in the race will at least make it more politically interesting
Mr Nader will be best-remembered for the central role he played in the dramas of the 2000 contest between George W Bush and Al Gore which still rankles with many Democrats to this day.
That contest hinged on a virtual dead-heat between the two main candidates in the state of Florida - which after a series of battles in the courts, eventually went the way of Mr Bush.
Democrats point out though that where only a few hundred votes separated Mr Bush from Al Gore, nearly 100,000 people voted for Mr Nader.
Those voters, say Democrats, would clearly have been likely to vote for Mr Gore had Mr Nader not been on the ballot - and so they blame Mr Nader for handing the presidency to George W Bush.
Mr Nader and his supporters do not buy that argument - and of course America is a democracy where the voters are entitled to vote for the candidate of their choice - but there is no doubt that his candidacy in 2008 will once again raise the whole issue of how tight presidential races between Democrats and Republicans can be affected by third party candidates.
Now very few races will ever be as tight as Florida in 2000, but there is no doubt that a third candidacy can help to determine who wins the White House.
Would Bill Clinton ever have become president in 1992 for example if Ross Perot had not run and taken around 19% of the popular vote?
Nader's candidacy is bound to change the themes of the campaign
It is reasonable to assume that at least some of those voters would have plumped for the Republican candidate George Bush senior if Mr Perot had not been around.
There are two ways in which third-party candidates can have a real impact.
First, like Mr Perot they can simply attract a huge number of votes.
Second, like Mr Nader in Florida in 2000, they can attract votes in a finely balanced race and thus effectively hold a kind of balance of power.
It seems reasonable to assume - on the basis of his track record - that Mr Nader is not going to get anywhere near the 19% of the vote that Mr Perot once achieved, so any influence he may have in 2008 will depend on the race being a desperately tight one again.
If it is, then Mr Nader's presence on the ballot may once again be highly significant.
The first candidate to discuss the Nader candidacy, Barack Obama, did not appear troubled by the prospect, though, merely noting that it was the job of a Democratic contender to be so compelling that the prospect of a few per cent of the vote being diverted to another candidate would make no difference to the outcome.
We should beware of treating the contest for the White House simply as a horse race - rather than as a battle for ideas
But we should beware of treating the contest for the White House simply as a horse race - rather than as a battle for ideas.
Ralph Nader once characterised the Democrat and Republican contenders for the presidency as "Tweedledum and Tweedledee" - implying that there was very little to choose between them.
Mr Nader's consumer-focused, environment-driven brand of radicalism ensures that a whole other set of ideas will once again get an airing this time around.
Even if he does not affect the outcome, the candidate who now replaces John McCain as the oldest man in the race will at least make it more politically interesting.