Their special relationship has been to their mutual advantage - until now. But the balance of power has changed since a nervous opposition leader met President Clinton for the first time in 1996. Mr Blair worried about whether to call him "Mr President" or "Bill". Mr Clinton said "Hi, Tony" and the two men never looked back: the Oxford- educated former lawyers also had much in common politically. "They get on very well, and it is a genuine meeting of minds," said one British minister. "In many ways they are very similar. But Tony does not have a zipper problem."
When Mr Blair returned to Washington as PM in February this year, the American media was dominated by the Monica Lewinsky affair. Mr Blair heaped praise on his friend, telling him at a White House dinner: "Bill, I am pleased to call you a good colleague and friend. I know I'm not alone in supporting you. I know the American people support you." They fended off questions about Miss Lewinsky after agreeing their lines in advance. But it may not be so easy for Mr Blair when he visits New York on September 21.
Yesterday, Downing Street insisted Mr Blair still regards Mr Clinton as a "very close friend and ally" who had done more than any other President to bring about peace in Ireland. Number 10 said the Prime Minister "hoped" to see Mr Clinton while he was in New York, although events are moving so fast in Washington that nothing can be taken for granted.
He will not want to abandon Mr Clinton in his hour of need, but Mr Blair has put light between him and his buddy when it suited him in the past. Modernisers including Mr Blair distanced themselves when Mr Clinton's first presidency ran into political troubles. Mr Blair quietly transferred his primary political affections to Paul Keating, Australia's Labor Prime Minister, before Clinton won a second term in 1996 and Mr Keating lost power.
Mr Blair drew two lessons: firstly, it is not enough just to win power - a party must know what it intends to do once in office. Secondly, as Mr Blair put it, politicians must avoid Mr Clinton's biggest mistake: "running on one basis and governing on another".
As well as learning from Mr Clinton's mistakes, Mr Blair always swaps notes with him on campaigning and, above all, retaining electoral support. That may seem a forlorn hope for the ailing President now.