It’s a phrase that strikes fear into the heart of every US political consultant and campaign manager: the ‘October surprise.’

Here, in order of their likelihood, are four events which would shake up the race and potentially decide who will be the next President of the United States of America. Nos. 5-7 to follow in Part Two.

1) Major Palin scandal

Given the apparent lack of vetting of Governor Palin by the McCain campaign, a fresh scandal or revelation about the 44-year-old is the most likely ‘October surprise.’ Indeed, there is one revelation we can already count on. The result of an independent investigation into ‘Troopergate‘ will be released on October 10. Should the report contain significant criticism of Palin, her anti-establishment, reformer credentials will be out of the window. This one is already worrying certain Republicans – they’re currently attempting to derail the investigation.

There are other potential scandals floating around too. The National Enquirer has run allegations that Palin had an affair. Normally, of course, the Enquirer isn’t taken very seriously, but since their success with outing the Edwards affair earlier this year the mainstream media is required to take a second look. Currently there is no evidence for the affair but should conclusive proof come to light expect the Republican base to fall out of love with the Alaskan Governor fast.

Any major Palin scandal would lead to questions about whether McCain will drop her from the ticket, Eagleton-style. That will depend on the nature of the scandal, and the proximity to the election and, whilst betting markets show that nearly 20% think there is a chance of that happening at the moment, it is very unlikely since it would almost certainly doom McCain’s chances of success. Regardless of whether he drops her, a Palin scandal would significantly harm the Republican ticket, leading to questions about McCain’s own judgement, as well as the electability of Governor Palin.

Would help: Democrats

2) New Osama bin Laden tape

Al-Qaeda has a habit of timing events to foreign elections. Whilst a terrorist attack on US soil would have a bigger impact, more likely is the release of a fresh video or audio tape from Osama bin Laden discussing the American election. He did exactly that in 2004, with the CIA’s own analysis suggesting it was intended to help the re-election of President Bush.

The release of a new bin Laden tape is anticipated and expected to the extent that some commentators have suggested that the absence of such a tape in October should be taken as demonstrating that he favours an Obama presidency. It’s unlikely that he does. After all, he wanted a continuation the Bush term in 2004, and a continued US presence in Iraq helps al-Qaeda recruitment and fund-raising. A tape would highlight fears that the Democratic nominee is not ready to stand up to terrorists, with the Republicans (as is traditional) enjoying a boost in votes.

Would help: Republicans

3) Obama assassination attempt

It’s been a major – and largely unspoken – fear since Senator Obama first announced his candidacy.

He received Secret Service protection earlier than any other Presidential candidate in history. Whilst the apparent assassination threat to Obama from the arrests in Denver turned out not to be credible, a genuine one, particularly late in the race, would be a major ‘October surprise.’

Assessing the impact of an attempt is not clear-cut, since the circumstances and nature of the event would be important factors. A foiled or failed attack would probably remind voters of the historic nature of Obama’s candidacy, and of the personal risk he was taking in running. Republicans would no longer be able to publicly suggest he is not patriotic enough and there would likley be an increase in sympathy votes. However, depending on the ideology behind the attack, it could also bring security issues to the fore, aiding the Republicans. Overall though candidates tend to gain when they suffer personally (as Obama’s failed 2000 bid for Bobby Rush’s congressional seat can testify). A failed assassination attempt would likely help Obama and might therefore spark all kinds of right-wing ‘he did it himself’ conspiracy theories.

Would help: Democrats, probably

4) McCain health scare

Taking inspiration from The West Wing, a health scare for the 72-year-old, cancer surviving, ex-POW Republican nominee is not beyond the realms of possibility. This should be a big fear for Republicans because any health scare significant enough to require disclosure to the public and/or time off the campaign trail will pretty much mean game over, especially if it’s fresh in voters’ minds come November 4. Expect a major drop-off in GOP votes if such an event forces voters to seriously consider whether Governor Palin is experienced enough to take over as President.

Would help: Democrats

Expect Part Two with Nos. 5-7 tomorrow.