ABOARD AIR FORCE ONE (Reuters) - President Barack Obama, former President George W. Bush and their wives shared Air Force One on Monday on a flight to South Africa, where they will be joined by two other former presidents at a memorial service for the late Nelson Mandela.
The rare gathering is a sign of the respect Mandela - who died on Thursday at age 95 - held among both political parties in the United States.
As the three-day trip to Africa began, the leaders were joined in a conference room on the plane by Attorney General Eric Holder, national security adviser Susan Rice, and Valerie Jarrett, a top adviser to Obama.
"I think it's a unique experience, and I think they all are remembering their different interactions with Nelson Mandela and his family," said Ben Rhodes, White House deputy national security adviser.
"He was a leader that intersected with so many different American political leaders of both parties over the years," Rhodes told reporters traveling aboard Air Force One.
Flying separately to Johannesburg were former Presidents Bill Clinton and Jimmy Carter. The only surviving former president not traveling was Bush's father, George H.W. Bush, who is 89.
Presidential travel usually requires weeks of preparation, but U.S. officials scrambled since Mandela's death on Thursday to put together logistics for the trip.
"Secret Service is used to, and prepared for, short-notice trips," said Brian Leary, a spokesman for the group designated to protect sitting and former U.S. presidents. He declined to comment on specific security measures for the trip.
Martha Joynt Kumar, a political scientist at Towson University in Maryland who studies the presidency, said the fact that four U.S. presidents were showing up underscored the importance of Mandela's life and legacy.
"It puts a ... stamp on the importance that the United States thought of Mandela, his importance as a world leader," she said.
The journey will likely be the most time that Obama and Bush will have spent in close proximity to each other.
Obama's wife, Michele, and her predecessor as first lady, Laura Bush, will be on the 16-hour flight, along with former Secretary of State Hillary Clinton.
"They're both grateful to be able to have the former president and first lady, and the former secretary of state on board," White House spokesman Jay Carney said.
The Bush camp was irked by frequent criticism during Obama's 2008 campaign, but over the years the two men have developed a cordial relationship. Obama offered gracious comments about Bush at the opening of the former president's library and museum this year.
The Obama White House has appreciated Bush's effort to avoid getting involved in the usual partisan rhetoric between Democrats and Republicans.
The 43rd president has a deep interest in Africa and a significant part of his legacy is the PEPFAR program that helps AIDS victims in Africa.
Obama was expected to deliver remarks at the service at Johannesburg's Soccer City stadium, where more than 70 leaders from around the world will commemorate the life of Mandela. He also will try to meet with Mandela's widow, Graca Machel, and some family members to pay respects, if time allows, aides said.
Obama likely will meet informally with South African President Jacob Zuma at the memorial, aides said.
The United States also planned to send a delegation to Mandela's burial on Sunday in Qunu, his ancestral home.
Obama met Mandela, and spoke with him occasionally, before he ran for president. He has said the South African leader's struggle against racism inspired him to become involved in politics.
The U.S. president did not meet with Mandela during a June visit to South Africa, as Mandela was gravely ill at the time, but Michelle Obama and daughters Sasha and Malia visited Mandela in 2011.
In June, Obama took his family to see the Robben Island prison cell where Mandela had been held, a "powerful experience" that Obama reflected upon as he crafted his remarks for the memorial service, Rhodes said.
In Washington, Vice President Joe Biden and his wife, Jill, signed a book of condolence on behalf of the American people at the South African Embassy. Biden wrote: "Mandela's head and heart lifted a nation to freedom. We will continue to keep his spirit alive and strive to live by his example."
South Africa's foreign ministry said Iranian President Hassan Rouhani would be at Tuesday's memorial in South Africa, although his name was not on an official list of attendees. Rhodes said it was not likely Obama would meet with Rouhani. (Additional reporting by Roberta Rampton, Tabassum Zakaria and Jeff Mason; Editing by Alistair Bell and Doina Chiacu)