Rev. Billy Graham had counseled US presidents since Truman

— -- The Rev. Billy Graham preached to millions of people around the world, but he was especially known for counseling U.S. presidents. He sat with many of them -- from Harry Truman to Barack Obama, and even met Donald Trump before his presidential run.

Graham had extraordinary access to the White House over the years and served as spiritual adviser to many of the presidents in their hour of need.

To Graham, it was not about politics, it was about unity and hope in times of crisis and national tragedy.

"My calling has been to help people look beyond this world and its problems to the world to come," Graham told ABC News in 2006.

But Graham was not always a White House favorite. Truman, the first of Graham's presidents, said he thought the young preacher was just a publicity seeker.

But perhaps because of such publicity, Graham's popularity grew.

By 1952, his words of encouragement helped convince Gen. Dwight Eisenhower to run for president and, once in office, establish a National Day of Prayer. Graham then became an Oval Office regular and a presidential golf partner.

It was a pattern that continued in the John F. Kennedy years. Though some Protestants weren't sure they could trust a Roman Catholic president, Graham liked Kennedy and helped put to rest the long-held suspicions.

"Kennedy, I met him because of his father. His father said, 'You know, the man that you ought to get acquainted with and get to know is Billy Graham,'" Graham said in 1997. "And he invited me to play golf with him, and we did get acquainted, and we became friends, and I like him very much."

After Kennedy's assassination, his successor, Lyndon Johnson, asked Graham to join him in prayer -- and an unlikely friendship developed between the clean-living pastor and the blunt-spoken politician. At Johnson's request, Graham spoke at his funeral.

Graham said of Johnson, "[He] was rough one side, but he was tender on the other ... and I think that he was sincere in his battle against poverty."

Graham had an especially close -- and complicated -- relationship with Richard Nixon. He stuck by Nixon's side through the Watergate scandal. But the reverend later said he was shocked to hear what was really going on in Nixon's secretly recorded White House tapes.

"I really had a deep affection for him, I felt like that I knew him, but there were things that apparently I didn't know," he said. "I've often wondered if there wasn't some strange demonic power that came into the whole White House system at that time."

Graham continued counseling presidents through the decades, remaining a White House fixture through the Gerald Ford, Jimmy Carter and Ronald Reagan administrations. He called Ford as he was making the decision about whether to pardon Nixon. Ford told Graham he had not yet made up his mind.

The reverend personally liked President Carter, who he noted was "one of the sweetest guys you could ever know."

Carter also had a deep admiration for Graham.

"He was broad-minded, he was innovative, he believed in breaking down the barriers between black and white when it was very unpopular to do so in the South," Carter said. "I just think that in almost every way, the things that he did as a Christian were admirable, and the kinds of actions that I have sought to emulate."

He had particularly kind words about Reagan, telling ABC News he was "the greatest. I mean, he helped turn this country around. He made us proud to be American."

When the President George H.W. Bush decided to enter Kuwait to repel Saddam Hussein's invading army, he sat up with him the night Desert Storm began. The elder Bush later honored him with the George Bush Award for public service in April 2006.

"When my soul was troubled, it was Billy I reached out to for comfort, advice, and prayer," Bush said as he gave Graham the award.

Graham was also one of several prominent clergies in President Clinton's circle of advisers.

"I doubt that many presidents ever wanted to be around him because they thought it would help them politically," Clinton said. "I think that they really felt and hoped that whatever the state of their own spiritual life, that by being with Billy Graham their own faith and understanding might be deepened."

He also counseled President George W. Bush, whom he had known most of his life.

"I've known him as a boy, I've known him as a young man, I've known him now still as a young man," Graham said. "And I'm very proud of him and I'm very thankful of the privilege of calling him a friend."

In 2010, Graham and son Franklin met with President Obama at Graham's home, chatting about wives, golf and Chicago, Obama’s adopted hometown.

Trump and wife Melania met Billy Graham at the reverend's 95th birthday party in 2013, but they never met after Trump took office.

Instead, Trump maintained a White House connection to the Graham family through Franklin Graham, who read a passage from the New Testament at Trump's inauguration and has attended at least one White House event during his administration.

Graham told ABC News' Diane Sawyer he was proud to minister to some of the most powerful men in the world.

"It was a great privilege for me and a great honor for me," he said.