Sharon's moves race beyond the long-dormant peace talks advocated by Bush and his predecessors, which presumed a series of reciprocal steps leading to final negotiations over the most complex issues.
"I think the Palestinian people get a huge amount out of this, and we hope they will capitalize on the opportunity that has been provided," said one U.S. official, who briefed reporters on the condition that he remain anonymous. He said the administration is ready "to engage very vigorously" to help the Palestinian Authority run Gaza effectively.
Israeli Prime Minister Ariel Sharon and President Bush leave after presenting a joint statement to the media.
(Pablo Martinez Monsivais -- AP)
Live, 3 p.m. ET: Ali Abunimah, co-founder of Electronicintifada.net and vice president of Arab American Action Network, will discuss Sharon's visit with President Bush.
Audio: Reporting from Jerusalem, The Post's John Ward Anderson discusses reaction Sharon's withdrawal plan.
Video: Palestinian politicians reacted angrily to Bush's statements.
Sharon proposes to withdraw 7,500 Jewish settlers from 21 Gaza settlements. He has also said he would close four small settlements with a total population of about 500 on the West Bank, where more than 200,000 Israelis have settled since Israel seized the land during the 1967 war. Far-right members of his governing coalition have objected to the withdrawals.
Sharon, who once directed Israel's aggressive settlement program, also wants Israel to retain control over six controversial settlements on the West Bank.
Naming them for the first time on Monday, Sharon said the communities must be strengthened for the sake of security. The settlements are Ariel, Maleh Adumim, Givat Zeev, Gush Etzion, Kiryat Arba and blocks in nearby Hebron.
In his letter to Bush, Sharon said Israel will speed construction of the security wall, but he accepted a U.S. position by saying it is "temporary rather than permanent, and therefore will not prejudice any final status issues including final borders."
The senior U.S. official told reporters that negotiators would have to resolve details later and said the president was not prejudging the outcome.
Bush said, "The realities on the ground and in the region have changed greatly over the last several decades, and any final settlement must take into account those realities and be agreeable to the parties."
The official noted that former president Bill Clinton and a host of experts have long concluded that the pre-1967 borders could never be reestablished, nor could all refugees return to Israel, a point acknowledged by various Palestinian negotiators.
Commentators said that, by backing much of Sharon's unilateral plan and declaring that refugees should expect to relocate to a future Palestinian state, Bush failed to offer much to the Palestinians. When negotiators were considering the future of Palestinian refugees in 2000, for example, Clinton discussed raising billions for Palestinians evicted from Israel since 1948.
Bush's endorsement changes the balance of future negotiations and "moves the line in favor of Israel," said University of Maryland professor Shibley Telhami. "This obviously is a violation of that longstanding American position that opens up a whole new debate."
Former Middle East negotiator Dennis Ross, calling Sharon's Gaza withdrawal "revolutionary," described Bush's endorsement as less than Sharon sought, but more than many in the U.S. administration had originally wanted to give.
Israeli Ambassador Daniel Ayalon said Bush's support gives Israel "the assurances to move forward with taking the risks which it's going to take by giving up territory, by evacuating settlements."
He added, "This is obviously an opportunity, and we hope that Palestinians would seize the moment."
Staff writer Mike Allen contributed to this report.