Let me briefly explain the whole conundrum that is the presidential primaries which may be unfamiliar to folks who are raised in the parliamentary democracy system of government. And also why I'm saying the serious business starts this fall (ie.around September 2007 onwards).
In the US, the Executive and Legislature branches of government are seperate entities with members of each to be elected independently. Unlike what's currently happening with Labour in the way they nominate Brown to succeed Blair, to secure the Democratic
party's nomination for the US Presidential elections, the candidate must basically amass enough support in the
series of primaries leading up to the Democratic National Convention held in August 2008 where the
party officially endorses the winner as its candidate for that particular Presidential elections.
In essence, it's a case of each man for himself(from soliciting funds to running their campaigns) for the primaries - the eventual Democratic nominee is the only one who will enjoy the backing of the entire
party and have the
party raise funds for his eventual election campaign in November. In theory, that's how it works.
Now, on to some practical issues people unfamiliar with American politics may not be aware of. Although the first Democratic primary starts in New Hampshire on 22nd January 2008 (the first caucus is actually held for Iowa on 14th January 2008), traditionally - at least in practice for the last 40 years, anyway - the real contender(s) is/are those who emerge with a strong showing
after Super-Tuesday in February. In essence, the caucuses and primaries can be viewed, for ease of understanding, as a "Democratic Presidential elections" in the same electoral college format(ie.number of "votes" for each state is dependent on proportional representation) which will grip the nation when the real elections come in November. Oftentimes, many successful, and usually heavyweight, Democratic nominees (regardless of whether they go on to win the elections in November), are not interested/focused on the first phase of primaries (ie.everything before Super-Tuesday). However, for the small fries, it's this very phase which will break their back if they haven't amassed enough support as many often run into problems with insufficient funding and without having performed well at this phase, chances of them pitting themselves against the heavyweights for Super-Tuesday effectively end. That will hopefully help enlighten some as to how John Kerry was able to outlast some of his more glamorous opponents and how Howard Dean - being widely touted as a Democratic hopeful way before the primaries got underway in the 2004 elections - was nowhere to be seen near the end.
In light of the number of heavyweights (whose reputations and widespread influence will no doubt gain them massive headstarts in terms of campaigning and raising funds to last the distance) who are contesting the Democratic seat this time, unlike 2004, I'd go so far as to say - in all probability -
Hillary Clinton, Barack Obama and John Edwards (and Al Gore if he runs) will be the only ones emerging after Super Tuesday with a realistic shot of securing the Democratic nomination.
The Republicans, on the other hand, would be subjected to much keener competition (something like the Democrats' primaries in 2004 IMO) and hence, it'd be tougher to call on that as there're no major heavyweights in there - although I'm hardly bothered what goes on in there being a Democrat myself

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In view of the above factors, it is therefore unlikely Al Gore will actually declare his intention to run for the elections any later than fall (truth be told, I'd expect him to do so by July if he's running) IMO given the massive campaign aides and funds he'd need to raise. In truth, I'd venture so far as to place my wager the moment I know - definitively - whether Gore decides to run.
From a betting perspective, the challenge here is finding the opportune moment to place a wager (regardless of who you fancy to win the nomination) whereby the odds are optimized in our favour - saying that, if we were to wait until after Super-Tuesday, we'd probably be looking at odds so dramatically shortened we wouldn't seriously consider any value in this market. As things stand, my personal take would be a toss between Hillary Clinton and Al Gore(assuming he runs) - however, for all the value on Gore, I'd hate to shoot myself in the foot by backing a non-runner (except, in this case, you don't get a refund of your wager)

but that's probably the risk that comes with the value.
For those who are looking for a safer bet, I'd personally advise backing the
next American President to be a Democrat - that's just me speaking on my convictions though. In truth, I've been following this market for more than a year now and the value on the Democrats has been depreciating by the day - prior to the Congressional elections, the Democrats were trading at evens; shortly after, it was 5/6 (or 1.83) and they're only 8/11(1.72) or 2/3(1.66) at most places now.