Boris v Ken or Brown v Cameron?

More than any other recent election, the race to be London mayor has become a personality contest. Electioneering has been highly personalised with "Ken" slogging it out against "Boris" and headline writers paying scant regard to the party machine behind them.

But the result on May 1st will be nevertheless significant to the Conservative and Labour leadership, not least because David Cameron is increasingly aligning himself with Boris' - now safely gaffe-free – campaign.

The London election marks the first significant test for the Westminster parties since the aborted autumn election. Although Boris' and Ken's respective bosses will no doubt blame the personality factor if their candidate is rejected, the allocation of seats in the Greater London Authority (GLA) will provide a useful barometer of public opinion before a general election in 2009 or 2010.

Many of the barriers to Mr Livingstone's re-election in May echo the challenges facing Mr Brown. Both men are vulnerable to a charismatic "change" candidate, a role being ruthlessly pursued by the Cameron and Johnson double act.

Mr Johnson has accused the incumbent mayor of being out of touch with the concerns of ordinary Londoners, an easy claim for an opposition candidate to make. But the fact remains that the man who first took City Hall as an independent candidate and bete noire of Tony Blair is vulnerable to claims of being part of the establishment, ironically from an Oxford-educated Old-Etonian.

The Conservative MP has in turn struggled initially to prove himself as a serious candidate fit for government. Prior to his campaign launch, pundits speculated on what one liners the notoriously gaffe-prone Tory would come out with and the Livingstone camp warned he would be a throwback to another era.

But Mr Johnson is widely acknowledged to have run a remarkably professional campaign, the effect of which has been magnified by the simultaneous media scandal surrounding alleged "corruption" in Livingstone's City Hall.

At the same time nationally the Conservatives have strove to "detoxify" their brand, with the effect voters are now happy to tell pollsters they are planning to vote Tory – leading to a double digit lead for Mr Cameron.

Senior Labour figures reportedly hope that if elected Mr Johnson will revert to type and undo much of this progress. No one is seriously suggesting Labour are deliberately abandoning Mr Livingstone in the hope Mr Johnson's tenure as mayor discredits the Conservative party, but it has been repeatedly spoken of as a conciliatory benefit of a win for Mr Johnson in May.

Mr Brown must call an election before May 2010 but governments invariably prefer not to drag out their term until the last possible minute. A 2009 election would give Mr Johnson just over a year to expose the Conservatives' inability to govern – a tough task given that Mr Cameron is reportedly prepared to draft in a team of experts to guide Mr Johnson through office.

The sense that Labour are now working on the assumption of a Johnson victory has been fuelled by Mr Brown's apparent distancing himself from Mr Livingstone's re-election bid. The Labour leader was notably absent from his campaign launch at Royal Festival Hall, although the pair have since been campaigning together.

Amid the growing suggestion Mr Livingstone may not win a third-term as mayor, Mr Brown seems unwilling to personally align his fortunes with the Livingstone campaign. Election watchers will remember last May's local and devolved government elections, when Mr Brown stayed visibly absent during Labour's string of defeats.

But with allegations of corruption surrounding City Hall Mr Brown may equally want to distance himself from Mr Livingstone's past as much as his future. The six-months following the bungled autumn election saw Labour rocked by a serious of high-profile scandals, including the resignation of one Cabinet minister, and the prime minister can ill-afford any further perception of sleaze.

In contrast to Mr Brown's hands-off approach, Mr Cameron has been increasingly appearing alongside his candidate. This is a recent development, with Mr Johnson receiving little endorsement from the Tory leadership until he had proven his mayoral abilities. The Conservative leader is undoubtedly stung by his experience of last summer's Ealing Southall by-election, where the candidate for "David Cameron's Conservatives" finished a poor third despite the party leader's frequent appearances in the constituency.

Unemployment for Mr Livingstone on May 2nd will further the perception that Labour is unable to retain seats in the south-east, increasing the argument Mr Brown is now haemorrhaging support from the aspirational voters who first voted Labour in 1997. A Conservative victory would in turn bolster the party's claim to be the government in waiting, with Boris at City Hall billed as the precursor to Mr Cameron's arrival in Number 10.

If faced with defeat next month Labour leaders are likely to deny this is the case and claim the public narrowly picked one big character over the other – before party strategists desperately try to analyse the impact City Hall will have on the outcome of the next general election.