After a long flight from Washington, Obama is to speak on Tuesday at the memorial service in an 80,000-seat soccer stadium in Johannesburg, where more than 70 leaders from around the world will commemorate the life of Mandela, who died on Thursday at age 95.

At his side will be his immediate predecessor, Bush, a Republican, as well as Democratic 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.

On board Air Force One for the cross-Atlantic flight to Africa were Obama, his wife Michelle, Bush and his wife Laura, and Hillary Clinton, who was Obama’s first secretary of state and who is contemplating her own run for the presidency in 2016.

The fact that the leaders from both parties joined together for the South Africa trip underscored the importance of Mandela’s life and legacy, said Martha Joynt Kumar, a political scientist at Towson University in Maryland who studies the presidency.

“It puts a … stamp on the importance that the United States thought of Mandela, his importance as a world leader,” Kumar said.

Air Force One stopped in Senegal’s capital Dakar to refuel on its way to South Africa. On the flight, the three families sat in a conference room toward the front of the plane, chatting to each other at a large round table.

Obama kept his usual quarters in the front of the plane, while the medical unit cabin was transformed into the Bushes’ quarters for the flight. Hillary Clinton stayed in the senior staff cabin.

It was Bush’s first flight on Air Force One since handing over power to Obama in 2009 and it was the most time he had spent in close proximity to Obama since returning to Dallas after he left the White House.

Carter and Bill Clinton will meet up with them in South Africa. Logistical issues prevented them from flying together.

SCRAMBLE FOR LOGISTICS

Presidential travel usually requires weeks of preparation. Preliminary work probably started a while ago as authorities would have recognized the gravity of Mandela’s illness, said Ralph Basham, former Secret Service director.

When Pope John Paul II passed away in 2005, President George W. Bush, Bill Clinton and George H.W. Bush attended the funeral.

“Where it’s really challenging is when a death and a funeral is unexpected,” Basham said. When Egyptian President Anwar Sadat was assassinated in 1981, the Secret Service decided it was too dangerous for the president and vice president to attend the funeral, so Former presidents Richard Nixon, Gerald Ford, and Carter went instead, he said.

“That was a real challenge because of the unusual situation there and not knowing who you could trust,” said Basham, who was lead Secret Service agent for Sadat’s funeral.

But the situation in South Africa is different. “You’re dealing with a sophisticated group of people in South Africa who have excellent law enforcement and military capabilities,” said Basham, now a partner at Command Consulting Group.

“They recognize that it would be an international catastrophe if something were to happen during these events,” he said.

A spokesman for the Secret Service declined to comment on specific security measures for the South Africa trip.

OBAMA TO PAY RESPECTS

Obama is expected to arrive in South Africa early on Tuesday.

Ben Rhodes, deputy White House national security adviser, told reporters that Obama in his 10-15 minute speech would talk about what Mandela meant to the people of South Africa and to him personally.

“His success wasn’t preordained. It had to be earned over a lifetime,” Rhodes said of Mandela.

Obama has said the South African leader’s struggle against racism inspired him to become involved in politics.

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.

Obama likely will meet informally with South African President Jacob Zuma at the memorial, aides said, and will try to meet with Mandela’s widow, Graca Machel, and some family members to pay respects, if time allows.

The United States also planned to send a delegation to Mandela’s burial on Sunday in Qunu, his ancestral home.

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.” DM

Photo: U.S. President Barack Obama (R) and former President George W. Bush attend a memorial for the victims of the 1998 U.S. Embassy bombing in Dar es Salaam, July 2, 2013. REUTERS/Jason Reed