WASHINGTON, Sept. 3 - Faced with one of the worst political crises of his presidency, President Bush abruptly overhauled his September schedule on Saturday as the White House scrambled to gain control of a situation that Republicans said threatened to undermine Mr. Bush"s second-term agenda and the party"s long-term ambitions.

In a sign of the mounting anxiety at the White House, Mr. Bush made a rare Saturday appearance in the Rose Garden before live television cameras to announce he was dispatching additional active-duty troops to the Gulf region. He struck a more somber tone than he had at times on Friday during a daylong tour of the disaster region, when he had joked at the airport in New Orleans about the fun he had had in his younger days in Houston. His demeanor on Saturday was similar to that of his most somber speeches after the Sept. 11 terrorist attacks.

"The magnitude of responding to a crisis over a disaster area that is larger than the size of Great Britain has created tremendous problems that have strained state and local capabilities," said Mr. Bush, slightly exaggerating the stricken land area. "The result is that many of our citizens simply are not getting the help they need, especially in New Orleans. And that is unacceptable."

The president was flanked by his high military and emergency command: Defense Secretary Donald H. Rumsfeld, Homeland Security Secretary Michael Chertoff and Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff.

As Mr. Bush spoke, Vice President Dick Cheney and Karl Rove, the president"s senior political adviser, listened on the sidelines, as did Dan Bartlett, the counselor to the president and Mr. Bush"s overseer of communications strategy. Their presence underscored how seriously the White House is reacting to the political crisis it faces.

"Where our response is not working, we"ll make it right," Mr. Bush said, as Mr. Bartlett, with a script in his hand, followed closely.

His speech came as analysts and some Republicans warned that the White House"s response to the crisis in New Orleans, which has been widely seen as slow and ineffectual, could further undermine Mr. Bush"s authority at a time when he was already under fire, endangering his Congressional agenda.

Mr. Chertoff said Saturday: "Not an hour goes by that we do not spend a lot of time thinking about the people who are actively suffering. The United States, as the president has said, is going to move heaven and earth to rescue, feed, shelter" victims of the storm.

The White House said Mr. Bush would return to Louisiana and Mississippi on Monday, scrapping his plans for a Labor Day address in Maryland. The rest of Mr. Bush"s schedule next week was in flux.

The White House also cancelled a major visit to Washington next week by President Hu Jintao of China. In a statement issued on Saturday, the White House said both Mr. Hu and Mr. Bush had agreed that "in the present circumstances, it was best not to have" the meeting, which would have demanded much of the president"s attention over the next days on growing difficulties between the United States and China over trade frictions, North Korea"s nuclear program and China"s military buildup.

The last-minute overhaul of the president"s plans reflected what analysts and some Republicans said was a long-term threat to Mr. Bush"s presidency created by the perception that the White House had failed to respond to the crisis. Several said the political fallout over the hurricane could complicate a second-term agenda that includes major changes to Social Security, the tax code and the immigration system.

"This is very much going to divert the agenda," said Tom Rath, a New Hampshire Republican with ties to the White House. "Some of this is momentary. I think the Bush capital will be rapidly replenished if they begin to respond here.

Donald P. Green, a professor of political science at Yale University, said: "The possibility for very serious damage to the administration exists. The unmistakable conclusion one would draw from this was this was a massive administration failure."

And Newt Gingrich, the former House speaker, urged Mr. Bush to quickly propose a rebuilding plan for New Orleans and the rest of the Gulf Coast, arguing that an ambitious gesture could restore his power in Congress.

"If it"s done right, it adds energy to the rest of his agenda," Mr. Gingrich said. "If it"s done wrong, it swamps the rest of his agenda."

The silence of many prominent Democrats in publicly criticizing Mr. Bush for his handling of the crisis reflects their conclusion that the president is on treacherous political ground and that attacking him would permit the White House to dismiss the criticism as partisan politics-as-usual, a senior Democratic aide said.

Scott McClellan, the White House press secretary, disputed the notion that Mr. Bush"s long-term political viability was endangered and said Saturday that he was confident that the administration would be able to push ahead successfully with its entire second-term agenda. "There are a number of priorities and we will address all of them," he said.