In August last year Pastor Kirbyjon Caldwell telephoned the White House and left a message for President Bush to call him back. When he did, Mr Caldwell told him that he had decided to endorse Barack Obama in this year’s presidential election.

“He took it very well,” Mr Caldwell said, though the call could hardly have thrilled the beleaguered President.

Mr Caldwell, a 55-year-old African-American, is not just head of America’s largest Methodist congregation, the 15,000-member Windsor Village United megachurch in Houston. He is Mr Bush’s religious adviser.

He introduced Mr Bush at the Republican National Convention in 2000 and gave the benediction at both Mr Bush’s presidential inaugurations. He is a close friend of the Bush family, has stayed at the White House, and officiated at the wedding of the President’s daughter, Jenna, in May.

In an interview with The Times this week Mr Caldwell bubbled with enthusiasm for Mr Obama, describing him as a family man with an extraordinary life story and “flat-out brilliant”.

He said he had never worked harder for a political candidate – advising him, introducing him to other evangelical leaders, helping him to break the Republican lock on the Christian vote. He made his first political donation to Mr Obama’s campaign. Each Friday morning he joins a conference call of up to 60 other religious leaders around the US who pray for Mr Obama – and sometimes for John McCain.

Mr Caldwell insisted his support had nothing to do with Mr Obama’s colour, and everything to do with his ability. “He stands the best chance of repairing America’s name around the globe,” he said. “He understands America can no longer afford to operate in a vacuum, making unilateral decisions without seeking partners.”

The sub-text of all this was Mr Caldwell’s disappointment with the last eight years of Republican rule, and his contention that Mr Bush’s well-intentioned presidency was hijacked by the Right after the terrorist attacks of 9/11.

“I refuse to throw him under the bus,” he said of his old friend, insisting he “was, and is, a good human being”.

The men met 11 years ago, when Mr Bush was Governor of Texas, and quickly became close. Mr Caldwell said that he agreed to introduce Mr Bush at the Republican convention in 2000 because he had helped minorities and the disadvantaged. In doing so, Mr Caldwell incurred the wrath of many African-Americans. “Some thought I was a traitor. Some thought I was being bought.”

What changed everything, Mr Caldwell argued, was 9/11. Policies to help the disadvantaged “fell off the table”. The war in Iraq “has not turned out well”.

Mr Caldwell was reluctant to describe his private conversations with Mr Bush, except on Iraq. “I don’t think America or the world understands how heavily the loss of life weighs on him,” he said.

Mr Caldwell laughs when asked if he will be invited to deliver the benediction at Mr Obama’s inauguration, but he has certainly helped make that inauguration more likely.

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