Obama will appear on Friday at the Saddleback church in Lake Forest, California, where at least 20,000 conservative Christians gather each week for services led by Pastor Rick Warren, the evangelical author of the bestselling inspirational book The Purpose Driven Life.

At first glance Warren and Obama appear the unlikeliest of allies — the conservative white preacher and the liberal black Democrat — yet aides to both confirmed last week that they have formed an intriguing friendship that may prove a key element in the next presidential campaign.

Fresh from their midterm election triumph, the Democrats have been energised by evidence that religious voters have deserted the Republican party in droves.

“I think the big story of 2006 is the support for Democrats by religious moderates,” said Professor David Domke of the University of Washington. “The [Republican] party is not the only game in town for Christian voters.”

Although Obama has yet to confirm that he will seek the White House in 2008, his visit to Warren’s mega-church is being widely interpreted as a move to establish his Christian credentials with a key group of voters.

The charismatic and articulate mixed-race son of a black Kenyan father and a white Kansas mother, Obama, 45, has soared into the Democratic presidential reckoning despite having spent only two years as a US senator from Illinois.

He first dazzled his party with an electrifying speech to its 2004 presidential convention and is now frequently described in the American media as the next Bill Clinton or even the next John F Kennedy.

Tommy Vietor, Obama’s spokesman, said the senator had met Warren when the preacher visited Washington last January. “They have become friends, speaking on the phone with some regularity,” he said.

The two men share an interest in Africa, which Obama visited last August. Warren, whose church runs an anti-poverty mission in Rwanda, will be hosting a two-day “global Aids summit” this week at which Obama will speak.

Also attending the summit will be Senator Sam Brownback, a conservative Kansas Republican who is considering a presidential run of his own. Brownback and Obama disagree about most social issues including abortion and gay marriage, but they have co-sponsored a bill calling for US action on the Sudanese region of Darfur and they will both take public HIV tests at Warren’s summit to encourage others to do likewise.

Obama is scarcely the first Democrat to reach out to conservative Republicans: Senator Hillary Clinton, his main rival for the Democratic presidential nomination, has had her own contacts with Warren and has won widespread praise in Washington for her collaborative efforts with rightwingers on Capitol Hill. She has also carefully tailored her pronouncements on sensitive issues such as abortion to avoid confrontation with the religious right.

Yet Obama has made a point of courting evangelical Christians. A speech he delivered to a Christian group at a Washington church last June was described by the Washingtonian magazine as “perhaps the most important dissection of the role of faith made by any Democratic politician in half a century”.

In a bestselling new memoir, The Audacity of Hope, and in numerous television interviews, he has urged his Democratic colleagues not to “avoid the conversation about religious values”.

He said recently: “I think we make a mistake when we fail to acknowledge the power of faith in people’s lives. We need to understand that Americans are a religious people. Substantially more Americans believe in angels than in evolution.”

There are already grumblings in Washington, however, where supporters of other leading Democrats carped that he also took a public HIV test on his visit to Africa and that this week’s repeat looked like a political stunt.

“Obama is like a Grand Marnier soufflé still rising in the oven,” one disgruntled Democrat told Newsweek magazine. “It will taste delicious when it’s finished, but it’s not there yet.”

Another Washington insider quoted Cyril Connolly, the English writer: “Whom the gods wish to destroy they first call promising.”

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