Barack Obama has done David Cameron more than a favour

The UK's friendship with America has been a unique source of pride, and sometimes panic in Number 10 for decades.

David Cameron and Barack Obama don't have the political intimacy of Ronald Regan and Margaret Thatcher, or the controversial common cause of George W Bush and Tony Blair.

But the president's willingness to speak so plainly, and powerfully, in support of staying in the EU is much, much more than a simple favour.

The most powerful politician in the world, could hardly have said more clearly that David Cameron is right, and his rivals are wrong.

Senior sources on the other side believe it's almost futile to try to match Remain's levels of support from the Establishment. But the Leave campaign has passion and enthusiasm that's rarely matched by their opponents.

And it is far from clear that the intervention, still two months away from the vote, will make very much difference. In the privacy of the polling booth, who will suddenly think of the American president when deciding which box to tick?

But Mr Obama's endorsement is a powerful emblem of the emerging early reality of the referendum campaign. The ranks of the powerful are assembling, for the most part, behind the case to Remain. The challenge for the Outers is to persuade us to ignore those siren calls.

'Slapdown'

PS. Downing Street is cockahoop tonight, not just because of President Obama's backing for the Remain campaign, but because of his elegant slapdown of Boris Johnson.

Mr Johnson raised eyebrows by referring to the president's Kenyan roots, questioning whether that meant he had Britain's best interests at heart. He also reminded Sun readers that Mr Obama had removed a bust of Churchill from the Oval Office, a story that didn't escape the British press at the time.

Mr Johnson's comments were branded offensive and insensitive by many but the president didn't resort to even mentioning them. Instead, he spoke about how much he loved Winston Churchill, and why he had moved the bust of the former prime minister to his private residence, where he sees it every day.

The president didn't just back the prime minister's case, but smoothly and - without breaking a sweat - took the most well-known leader of the Leave campaign down a peg or two.