Something unusual happened at the White House during a visit by David Cameron, the British prime minister.

Cameron and US President Barack Obama put on a forceful united front, but not against another country - this time, against the US Congress.

Cameron admitted that he was lobbying members of the Senate, pressuring them not to pass legislation that would impose sanctions on Iran if the negotiations over its nuclear programme break down.

That in itself was highly unusual.

For his part, Obama had his most forceful warning yet: he told members of Congress he would veto their bill.

He went on to warn members of his own party that if they tried to override his veto, he would take the case to the American people.

His argument is that they would be choosing war over the possibility of a peaceful solution. In this day and age, that would be a powerful argument for the war-weary American public.

He is unlikely to deter the now Republican-controlled Senate. They argue that the threat of more sanctions will force Iran to make concessions.

The Obama administration says what it would do is make the Iranians walk away, fracture the coalition and leave the blame for it at the feet of the US.

Obama put the odds of actually reaching a deal at less than 50/50, but he is telling Congress they better let him give it or try - or else he will tell the American people they are to blame for what comes next.