For nine days, as European and United Nations officials have called urgently for a cease-fire in Gaza, the Bush administration has squarely blamed the rocket attacks of the Palestinian militant group Hamas for Israel’s assault, maintaining to the end its eight-year record of stalwart support for Israel.

Mr. Bush, in his weekly radio address on Saturday, said the United States did not want a “one-way cease-fire” that allowed Hamas to keep up its rocket fire, and Vice President Dick Cheney on Sunday echoed the point, declaring that only a “sustainable, durable” peace would be acceptable.

Many Middle East experts say Israel timed its move against Hamas, which began with airstrikes on Dec. 27, 24 days before Mr. Bush leaves office, with the expectation of such backing in Washington. Israeli officials could not be certain that President-elect Barack Obama, despite past statements of sympathy for Israel’s right of self-defense, would match the Bush administration’s unconditional endorsement.

“Obviously Bush, even by comparison with past U.S. presidents, has been very, very pro-Israel,” said Sami G. Hajjar, a longtime scholar of Middle East politics and a visiting professor at the National Defense University. “Despite Obama’s statements, and his advisers who are quite pro-Israel, the Israelis really didn’t know how he’d react. His first instinct is for diplomacy, not military action.”

Mr. Hajjar said that in addition to relying on the backing of Mr. Bush, Israeli officials may not have wanted to begin their relationship with the new president by forcing him to respond to their military action. On Dec. 19, just one month before Mr. Obama’s inauguration, Hamas declared an end to an Egyptian-mediated truce with Israel that had taken effect in June, and rocket attacks from Gaza have been increasing since then.

Mr. Obama has disappointed many commentators in the Muslim world by steadfastly declining to condemn the Gaza operation, and he maintained his silence over the weekend as Israel began a ground invasion. “President-elect Obama is closely monitoring global events, including the situation in Gaza, but there is one president at a time,” said Brooke Anderson, chief national security spokeswoman for Mr. Obama, repeating what has become a mantra for the incoming administration.

Mr. Obama’s stance has been interpreted by Hamas spokesmen and others as tacit assent both to Israel’s actions and to the Bush administration’s policy. Aides to the president-elect say he has spoken with President Bush and Secretary of State Condoleezza Rice about the situation in Gaza but they do not expect him to carve out a distinct position before his inauguration.

In the meantime, Mr. Bush and Mr. Cheney, while expressing concern about the deaths and suffering of Palestinian civilians, have offered unqualified understanding for Israel’s assault.

In an interview Sunday with Bob Schieffer of the CBS News program “Face the Nation,” Mr. Cheney said Israel “didn’t seek clearance or approval from us” before beginning a ground invasion on Saturday. But he said that in previous discussions, Israeli officials said they would respond forcefully to rocket attacks, “and if they did, they would be very aggressive in terms of trying to take down Hamas.”

Asked whether sending troops into Gaza was a mistake, the vice president replied that “it’s important to remember who the enemy is here,” adding, “You haven’t had a conflict between two U.N. charter-member states, you’ve got a U.N. member state being attacked by a terrorist organization.”

Leading Democrats, including Senators Harry Reid of Nevada and Dick Durbin of Illinois, both of whom appeared on programs on Sunday, have also expressed support for Israel. “I think this terrorist organization, Hamas, has got to be put away,” Mr. Reid said on NBC’s “Meet the Press.”

The fighting between Israeli troops and Hamas comes as a dismal coda to the Bush administration’s second-term push for Middle East peace, which has occupied much of Ms. Rice’s tenure as secretary of state. For Mr. Obama, who flew to Washington on Sunday to join his family, it adds a new crisis to an agenda already packed with challenges, beginning with the economy.

During his presidential campaign, Mr. Obama promised a new, positive approach to the Muslim world, including “America Houses” offering English lessons in Muslim countries and an “America’s Voice Corps” to spread the truth about American values. Mr. Obama’s aides have said he will unveil the new approach with a speech in a Muslim capital during his first 100 days in office. But Israel’s invasion of Gaza, and Mr. Obama’s studied silence about it, threatens to short-circuit his plans for an American image makeover.

Critics abroad and at home have noted that Mr. Obama’s “one president at a time” policy did not prevent the president-elect from speaking out against the terrorist attacks in Mumbai, India, in November, when he condemned what he called the “hateful ideology” of militant Islam.

In the absence of any new statement, many have recalled Mr. Obama’s remarks last July in the Israeli town of Sderot, where he implicitly recognized Israel’s right to respond militarily.

“If somebody was sending rockets into my house, where my two daughters sleep at night, I’m going to do everything in my power to stop that,” he told reporters. The Israeli defense minister, Ehud Barak, quoted Mr. Obama’s statement in justifying the attack on Hamas during a news briefing on Dec. 29.

Martha Joynt Kumar, a political scientist at Towson University who studies presidential transitions, said Mr. Obama’s predicament exemplified the treacherous weeks between election and inauguration, and the way inspiring visions inevitably give way before unexpected events.

“On a campaign, you control what you talk about and when you talk about it,” Ms. Kumar said. “When you begin governing, you have to respond to what happens in the world.”