The timing of the administration's major policy shift on Cambodia, although months in planning, was dictated largely by domestic politics and concern that congressional support was fast eroding for what has been U.S. policy for the past decade.

The decision, made in the secret and intensely political style that has become the administration's hallmark, was made Friday afternoon in a meeting attended only by President Bush, Secretary of State James A. Baker III and White House national security adviser Brent Scowcroft, sources said.

The timing of Bush's decision, made after months of internal discussions over the policy among White House, State Department and other administration officials, "was dictated by Hill politics," one senior official said. It was aimed at ending the erosion of support in Congress for the U.S. policy of giving aid to two noncommunist Cambodian resistance groups headed by former ruler Prince Norodom Sihanouk who are allied with the Khmer Rouge.

Several high-level White House and State Department officials apparently were unaware of the precise change agreed to in the longstanding policy until early this week. The administration did not tell its major allies in Southeast Asia until Tuesday afternoon and it did not alert its congressional supporters until early yesterday morning.

Assistant Secretary of State Richard Solomon, in Paris for a meeting on Cambodia of the Permanent Five U.N. Security Council members, was notified formally in a cable sent Tuesday evening, sources said. Solomon briefed his Chinese counterpart yesterday morning.

Representatives of the six-country Association of Southeast Asian Nations (ASEAN) were upset by the U.S. move, the official said. Several representatives from the ASEAN six -- Brunei, Indonesia, Malaysia, the Philippines, Singapore and Thailand -- are staunchly pro-Sihanouk or anti-Vietnamese. In addition, Washington has maintained that its Cambodia policy reflects the wishes of its allies in the region. "This time we told them what we are going to do," the official said. "But they are going to have to move with us . . . they can't split with us now."

Administration officials said yesterday there had been no change in the goal of an overall settlement to the 11-year-old war in Cambodia through free elections.

The announced changes, they said, had been drafted and reworked for months by Robert Kimmitt, undersecretary of state for political affairs. Elements of the new approach, one source said, had been "shot down" before by Scowcroft and his senior aides.

But Baker and several close aides had been growing more and more uncomfortable as negotiations in Paris stalled, and the Khmer Rouge's battlefield and political strength grew while the Phnom Penh regime continued to weaken.

If the Khmer Rouge, believed responsible for the deaths of more than one million Cambodians from 1975 until Vietnam invaded in 1978, won and the killing started again, the political blame likely would fall on the administration, several administration officials have said.

Congressional support for the policy of overt and covert aid to the noncommunist resistance, headed by Sihanouk, was weakening, especially in the Senate. The Senate intelligence committee voted last month against any further covert aid and a bipartisan group of 11 influential senators circulated a letter to their colleagues last week asking Bush to change the policy.

Finally, critics of the policy, growing in number since Vietnamese troops withdrew last fall and after a highly critical ABC-TV documentary in April, said aid to Sihanouk was only helping the Khmer Rouge by diverting Hun Sen's forces.

Baker, when he outlined the Cambodia policy proposal at the White House on Friday, cited eroding congressional support as another argument in favor of change. This time Bush agreed, sources said, although he did not agree with a Baker suggestion to open direct talks with the Hun Sen regime for the time being.

One senior aide, explaining Friday's move, said, "The Hill was getting restless. That gave it {the effort to alter the policy} an extra added push." After the Senate intelligence committee vote and with the House intelligence committee set to vote this week, the aide said, "we said there is no better time to do this. Kimmitt had been trying to do this for a couple of months . . . trying to bring the bureaucracy along. We said, 'Let's just do it.' "

The administration hopes the moves, several of which had been suggested by the 11 senators last week, will "buy some time," as two senior officials said, to see if a negotiated settlement can be reached.

Asked why Baker's move for direct talks with the Phnom Penh government was shelved, one official said "we're hedging our bets on that one. Baker would like to do that, to deal with Hun Sen directly" but "we're looking for a favorable response" to the latest moves. "I think it will happen sooner rather than later," the official said.

Another senior official said "we are considering direct negotiations with Hun Sen but we don't want to give him something that could be seen as a blessing that would give him a leg up in the elections process."

A third official said such direct talks could occur "only if it could be done in a manner to advance us toward our goal of free and fair elections in Cambodia."

Rep. Stephen J. Solarz (D-N.Y.), the administration's chief ally in the House, called the move "a welcome and constructive adjustment which should help solidify support on the Hill for continued assistance to the noncommunist resistance." He said the "change in policy . . . will significantly advance the prospects" for ending the war "in a way that will effectively prevent the Khmer Rouge from battling their way back into power."

But Rep. Chester G. Atkins (D-Mass.), a leading critic of the policy, said the move was "only a first step and will not make any difference until or unless Secretary Baker is willing to terminate" assistance to the noncommunist resistance.

Atkins said the administration "finally recognized that support for the coalition at the U.N. is undermining our ability to" stop the Khmer Rouge. That move "eliminated the rationale for the aid program," he said.