Commentators are calling it the most open race in more than 50 years. With no sitting president or vice-president in the running, next year’s US presidential election is already attracting a lot of attention: 21 months out is unusually early even for the US, with its notoriously long campaigning season.

So far there are six serious candidates: Democrats Hillary Clinton, John Edwards and Barack Obama, and Republicans John McCain, Mitt Romney and Rudolph Giuliani.

While it’s not impossible that one of the other declared or semi-declared candidates could make a surprise showing, at this stage they seem to be too remote from the mainstream or too far behind in fundraising to challenge the big six.

Giuliani, the latest entrant, has not made (or even scheduled) a formal announcement, but there’s little doubt that he’s running. Appearing on Fox News Monday evening, he said “We still have to formally announce it and do a few more things”, but emphasised, echoing Hillary Clinton, “I’m in this to win.”

As the madness of Iraq shows no signs of letting up, it’s shaping as a bad year for Republicans. McCain, formerly the GOP frontrunner, has lost ground in recent weeks with his support for the Bush plan for escalating the war.

If Giuliani can put a decent campaign together over the next few months, he will probably establish himself as the only Republican with a real chance of beating his fellow New Yorker Clinton.

Conservatives are spitting chips at the prospect of being saddled with either McCain or Giuliani, both of whom are regarded as insufficiently anti-choice and anti-gay. Romney is closer to their views, but his years as governor of Massachusetts failed to show a hard-line conservative record. He’s also a Mormon, which makes him suspect among some evangelical Christians — at least the few who still care about theology.

Clinton, of course, has her own problems, not least her failure so far to apologise for her initial support of the Iraq war. But her huge advantages in fundraising ability and name recognition will make her hard to beat for the Democrat nomination.

The major unknown at the moment is whether any other big names will throw their hats in the ring. Two that could really make a go of it are Democrat Al Gore and Republican Newt Gingrich, but they would want to move soon.