Six members of the Senate Foreign Relations Committee have agreed to participate in a new process for weeding out unqualified nominees for U.S. ambassadorships, it was announced on Capitol Hill yesterday.

The process, announced by Sens. Charles McC. Mathias Jr. (R-Md.) and Claiborne Pell (D-R.I.), is patterned on the procedure that the American Bar Association uses to screen nominees for the federal judiciary.

In this case, the screening group will be the American Academy of Diplomacy, which was established early last year by 68 prestigious former U.S. diplomats, both career and noncareer. Academy President David D. Newsom, former undersecretary of state for political affairs, said the screening would be supported by a $100,000 grant from the Ford Foundation.

Newsom said committees organized by retired ambassador William E. Schaufele will try to interview nominees for ambassadorships and will report to the senators on the nominees' qualifications. The goal, Newsom said, will be to have three former diplomats on each committee: one noncareer appointee of a Republican administration, one noncareer appointee of a Democratic administration and one former career diplomat.

Initially, the reviews will be limited to those who have not headed a U.S. mission overseas, Newsom said.

Mathias, who has complained about the dubious qualifications of some political appointees, said, "Over time, we hope this procedure will raise the level of competence of ambassadors. This should bring a tangible boost in the morale of the career Foreign Service."

In October 1981, the American Foreign Service Association criticized the first 81 of President Reagan's ambassadorial appointments, saying too many of them were political appointees and "relatively undistinguished as public figures." Inevitably, it added, "such appointments lower the respect of foreign countries for the United States and make a mockery of the careful selection and long and varied experience which professional careeer officers bring to senior assignments within the service."

In a joint statement, Mathias and Pell said the new process "will not diminish the president's prerogative in nominating ambassadors. It will not reduce the Senate's responsibility in confirming ambassadors.

"What it will do is to assure that there is a more thoughtful and thorough consideration of the qualifications of particular candidates for particular posts," the senators said.

Secretary of State George P. Shultz, asked about such a screening process at a hearing of the committee last Friday, opposed the idea. Shultz said it is the "constitutional duty" of the president to nominate candidates for such posts and the Senate's duty to consider confirmation. Shultz saw no role in the process for a private study committee.

Joining Mathias and Pell in supporting the review process were Sens. Alan Cranston (D-Calif.), Christopher J. Dodd (D-Conn.), Daniel J. Evans (R-Wash.) and Nancy Landon Kassebaum (R-Kan.) Mathias said the senators do not pledge to vote against ambassadorial nominees if the screening group finds them to be unfit.

He added, "We'll deal with these recommendations just as we deal with American Bar Association recommendations on the Judiciary Committee."

Newsom said there is "much less agreement" on the necessary qualifications for an ambassador than for a federal judge. Nonetheless, he said there is a basis for reporting on the qualifications of ambassadorial nominees in a way that could be helpful to senators who have to vote on their confirmation.