A bipartisan group of senators, led by Majority Leader George J. Mitchell (D-Maine), yesterday asked President Bush to drastically change the administration's policy in Cambodia to ensure that the radical Khmer Rouge faction does not take over the country.

In a letter, the senators called on the administration to "open direct contacts" with the Vietnamese-installed Hun Sen government, to loosen restrictions on humanitarian and development aid to the country and to vote to vacate the United Nations seat now held by the Khmer Rouge in a coalition with two U.S.-backed, non-communist groups led by former ruler Prince Norodom Sihanouk.

The letter, signed by several of the most pivotal senators on the issue, reflects a growing congressional disenchantment, especially in the Senate, with the administration's policy of backing Sihanouk. The Senate Select Committee on Intelligence voted June 28 to cut off covert aid to Sihanouk and a second non-communist group, and it is not clear whether the Senate will go along with a $7 million program in overt aid approved by the House two weeks ago.

The senators' recommendations would force a major change in the administration's strategy, which has been to back Sihanouk and to isolate the Hun Sen government from international recognition and aid.

The letter was signed by Mitchell, intelligence committee Chairman David L. Boren (D-Okla.), Appropriations Committee Chairman Robert C. Byrd (D-W.Va.), Foreign Relations Committee Chairman Clairborne Pell (D-R.I.), Sen. Alan Cranston (D-Calif.), Sen. Bob Kerrey (D-Neb.) and Sen. Charles S. Robb (D-Va.), a key moderate whose support has been seen as critical for the administration's approach.

Four moderate Republicans, Sens. John C. Danforth (Mo.), William S. Cohen (Maine), Mark O. Hatfield (Ore.) and Nancy Landon Kassebaum (Kan.) also signed the letter.

There have been numerous reports -- and statements from Sihanouk and other non-communist leaders -- that Sihanouk's forces cooperate with the Khmer Rouge, who ruled the country from 1975 to 1979 and are believed responsible for the deaths of more than 1 million Cambodians. The senators expressed concern about those reports and asked the administration to "thoroughly investigate," because it is illegal for the United States to directly or indirectly aid the Khmer Rouge.

But critics also charge that any support for Sihanouk helps the Khmer Rouge by diverting and weakening Hun Sen's forces. The letter did not say that aid to Sihanouk should be withdrawn or imply that the United States should aid Hun Sen, but it said that "because we believe that the Khmer Rouge represents an unacceptable threat to the people of Cambodia," U.S. policy should be based "first and foremost upon preventing the return of the Khmer Rouge."

"We recommend that the administration open direct contacts with the Hun Sen regime in Cambodia and begin easing Cambodia's isolation by loosening restrictions on humanitarian and development aid and by allowing greater travel and communications between our two countries," the senators wrote.

The administration, which has asserted that it also wants to prevent a return of the Khmer Rouge, has insisted that a negotiated settlement among all the parties leading to U.N.-supervised elections offers the best chance for keeping the Khmer Rouge, by far the most effective fighting force in the country, out of power.

The fifth round of discussions among the permanent five members of the U.N. Security Council on Cambodia begins next week in Paris, but administration officials say those talks are stalled. Some administration officials, noting that problem, the increased congressional pressure and the continued Khmer Rouge gains, both politically and on the battlefield, are beginning to consider policy options that might lead to agreement on elections.