Editorial: A rivalry Mr Cameron will struggle to contain

Taken at face value, there was little in Boris Johnson's speech to the Conservative conference in Birmingham yesterday to disturb the Prime Minister's calm. A model of congenial disinterestedness, the London Mayor could hardly have been more on-message.

He declared outright that David Cameron will win the next election. He took his former schoolfellow's "blond-haired mop" jibe as cue for an extended metaphor with the Prime Minister as broom and the Conservatives once again called upon to clean up Labour's mess. He even used his paean to London and his own mayoral achievements as an exemplar of Tory policy on everything from free schools to an export-led industrial strategy. Tough on crime, supporting hard-working "strivers", bearing down on public sector waste: it was all there.

But it was not the substance that was dangerous for Mr Cameron. It was the delivery. And when it comes to delivery, there are few as good as Boris. Yesterday was no exception. With his trademark mix of eloquence and excess, bombast and bumbling, he wooed the Tory faithful with devastating success. "I think I've told you this before," he said, conspiratorially, as though picking up threads of a conversation with an old friend. The audience loved every minute of it.

Nor was Mr Johnson's appearance at a fringe meeting the previous evening any less of an event. Again, he gave full-throated backing to the Prime Minister. But from the scrum of adoring fans who greeted him at the railway station, to the standing ovation, to the "Mission Imborisable" promotional video, Mr Johnson looked more star turn than support act.

In an apparent effort to quell speculation, he has (albeit equivocally) dismissed talk of leadership ambition. Meanwhile, naysayers point out that running London is a far cry from running the country, and that it takes more than a sprinkle of pizzazz to turn even the canniest buffoon into a credible prime ministerial candidate. Perhaps. But it hardly matters.

Slick, presentable, a former PR executive; Mr Cameron is no mean performer himself. Yet he is a pale moon to Mr Johnson's bright sun. With the Government in the mid-term doldrums, little but further austerity to look forward to, and Boris sounding like a man who can lead the party (and by extension the country) into the sunlit uplands of a crime-free, enterprise-driven, small-government future, is it any wonder that tongues are wagging?

Thus far, Mr Cameron is playing it cool. Notwithstanding the odd barb – "Boris will be Boris" hardly characterises a serious contender for Downing Street – the Prime Minister has shrugged off the tittle-tattle and maintained appearances of collegiate affability. It is the only sensible strategy, and, for the time being, the danger from Boris is containable.

The road ahead may be rather bumpier, however. It is far from inconceivable that, in the run-up to the election, were the economy still struggling for growth and the Tories still struggling in the polls, the clamour for Boris might just become deafening. Indeed, his mere existence as an alternative is enough to wreak havoc for Mr Cameron and his party alike.

As the Prime Minister takes to the stage in Birmingham this morning, he has any number of obstacles to overcome. He must steal back the political initiative from the opposition. He must reassure an increasingly sceptical public that his economic strategy is the right one. He must blow away the dangerous scent of dilettantism that has spread over him and his Government in recent months. Ultimately, though, the biggest challenge of all may come from the "blond-haired mop" within.