Boris Johnson cleared his Mayor of London diary to go and help win votes in Eastleigh for the Conservatives today, with Prime Minister David Cameron still thousands of miles away in India.

The media-savvy Mayor hit the campaign trail in support of Maria Hutchings - the party's candidate who has launched a second campaign to convince Eastleigh voters to pick her for parliament, after Chris Huhne resigned in disgrace earlier this month.

Johnson's attendance in the Hampshire constituency may put rocket fuel into Hutchings' campaign. Located outside Southampton, the area has no record of being insulted by the outspoken London Mayor, which should help his cause.

Meanwhile, Cameron is talking jet fighters in India, cut off from the grimy business of grinding out a by-election win in tough conditions.

But isn't it worth considering who benefits from Johnson attending the Eastleigh campaign?

Hutchings could certainly benefit from having the nation's favourite celebrity politician tramping from door to door in her name. Johnson's endorsement is valuable because he is one of that rare breed of politicians popular with people who don't like politics.

But Johnson can also strengthen his own reputation as a vote-winner for the Tories. If he manages a gaffe-free appearance, his visit could refuel speculation about a challenge for the party leadership.

For a period last year, in the sun-dappled aftermath of the Olympic Games, it seemed more a case of when, not if Johnson, would depose his Eton chum Dave as Prime Minister.

When Johnson dazzled with a speech to thousands in Hyde Park on the eve of Games, Cameron was forced to sit and watch as his old friend repeatedly failed to entirely dismiss a leadership campain.

Fast forward to now, and at a time of increasing disquiet among Tory MPs about the continuing failure to revive the economy by Chancellor George Osborne, Johnson's credentials as a bona-fide vote winner remain intact.

Winning in adverse conditions for the Conservatives is something Johnson showed he can do, by wresting City Hall from Labour in 2008 and then defending from Ken Livingstone last year. Those two victories were notable achievements, considering large swathes of the capital city are Labour strongholds.

Being a vote winner is a very attractive quality indeed among Conservative MPs, many of whom fear being punished by voters at the 2015 General election for the enduring economic chill, which it is hitting living standards and squeezing household budgets.

Going to Eastleigh also draws attention to another undulating narrative - the leadership of David Cameron.

The Prime Minister's leadership has come in for criticism of late as being distant and aloof. His relationship with the grassroots is in need of some tidying up, while the recent gay marriage vote showed how few favours the PM can count on from his own backbenches.

It was left to Equalities Minister Maria Miller to introduce the gay marriage Bill in the Commons. This despite the Prime Minister being personally linked by conviction to the legislation, which was not to be found in the party's 2010 manifesto.

Previously, Johnson appearing in Eastleigh would have been no problem for Cameron; a convention that Prime Ministers do not get involved in by-elections would have neutralised the issue. Unfortunately Cameron himself broke that convention, by visiting Corby during the by-election there last year - after chick-lit author Louise Mensch's shock resignation.

As a result, the voters are practically guaranteed a visit by the Prime Minister when he returns from India tomorrow (Thursday). But he will be trailing in Johnson's wake.

Johnson's visit could remind everyone of his own strengths as a popular performer. Meanwhile, Cameron will be conspicous by his absence,

With recent figures suggesting the country's economic malaise will last until the next election and beyond, Johnson's visit to Eastleigh comes at a very tricky time for the Tories' embattled leader.