Farage hopes he is on his way to Westminster Dylan Martinez/Reuters

They are 'The Untouchables' of British politics. No matter how much muck is thrown at them or how many gaffes they blunder into, nothing ever sticks to Nigel Farage and Boris Johnson.

Both have now chosen the seats they believe most likely to send them to Westminster next year, and the prospect of their election has been greeted with equal measures of delight and horror.

Supporters see them as clever, straight-talking and effective politicians of the sort the nation is crying out for. Critics brand them dangerous, egotistical buffoons who shouldn't be allowed anywhere near power.

Both individuals certainly have something, whether it is charisma, celebrity or an ability to appear human, that allows them to reach people from all political persuasions that other politicians can't get near.

Both also share the ability, and often the desire, to give David Cameron sleepless nights. But which of them poses the greatest threat to the prime minister and the Tory party?

Clearly Nigel Farage and his Ukip army is the immediate electoral threat. Their success in the European and local elections, along with recent polls, show they are a force to be reckoned with, particularly in Tory areas.

boris and cameron
Boris Johnson could be David Cameron's worst nightmare at 2015 polls AFP

And, despite Farage's well-founded claim that Ukip appeals as much to disillusioned Labour voters as Tories, it is still the Conservatives who have most to lose by any serious surge in support for the party in 2015.

Internal polling leaked to Sky News has revealed that the party will concentrate its efforts into nine Tory seats in the election campaign compared to two Liberal Democrat enclaves and just a single Labour constituency.

Farage has already stated he wants to throw all his party's resources at a couple of dozen seats in an effort to maximise his chances of returning a few candidates to Westminster, so Labour isn't entirely in the clear.

In reality, Farage will be delighted if he wins in South Thanet and over the moon if Ukip secures one or two more MPs. But the big danger is that he will do well enough in key seats to split the Tories and hand Labour victory.

That is precisely the argument Johnson has been making. And the London mayor is seen as the Tories' Eurosceptic, not-so secret weapon against Ukip.

However, MP Farage, even with a couple of Ukippers around him, would be a far less worrying prospect. The House of Commons has a habit, for good or ill, of swallowing up and smothering mavericks, independents and fringe parties.

Just ask George Galloway, Martin Bell and even the Green's Caroline Lucas who made a similar breakthrough in 2010.

The biggest threat would be if Cameron failed to keep his Eurosceptic MPs onside and they started eyeing up Ukip as possible allies or even an alternative home.

That could become a real problem if Cameron failed to win a strong majority at the next election or failed to convince his party he had negotiated a reasonable new relationship with the EU.

That is also where Johnson becomes a real danger. Once selected as a candidate, he will be inside the Tory tent and will be expected to be scrupulously loyal.

That doesn't mean he won't occasionally do something daft and annoy his masters. His recent suggestion that any Brit travelling to Iraq or Syria should immediately be branded a terrorist and presumed guilty is just such an embarrassment.

But the public's readiness to chalk these things up as "just Boris being Boris" will insulate him and the Tories to a great extent.

But if Cameron again fails to win a majority in 2015 a leadership challenge from MP Johnson would be a virtual certainty. But in those circumstances the prime minister would probably be forced to stand down anyway, irrespective of any threat from the mayor.

Should Cameron win that majority he would expect the support and loyalty of his party for some time, probably until he chose to stand down. Unless, and it is a big unless, he fails to convince with his EU re-negotiations.

Once again, it would be the good old Tory civil war that would hand both Johnson and Farage their biggest opportunities and Cameron his greatest nightmare.