Senate Republican and Democratic leaders, after weeks of wrangling, have worked out a tentative agreement on committee assignments that would weaken GOP control over most major committees, including Armed Services and Foreign Relations, Senate officials said yesterday.

However, sources emphasized that the agreement between Senate Majority Leader Robert J. Dole (R-Kan.) and Minority Leader Robert C. Byrd (D-W.Va.) was being fine-tuned and could fall apart before being unveiled, possibly next week.

"It's still a little squishy," one source said. "Tentatively most people have assignments, but we have to shift some around."

If approved, the agreement would leave Republicans with only a one-vote margin on eight of the 12 major committees, thus improving the chances of the Democrats on those panels to block or significantly alter legislation.

Under the agreement, the GOP would continue to have a two-vote margin on four major committees -- Energy and Natural Resources, Finance, Judiciary and Labor and Human Resources.

The eight major committees on which the Republicans would have a single-vote margin are Agriculture; Appropriations; Armed Services; Banking, Housing and Urban Affairs; Commerce, Science and Transportation; Environment and Public Works; Foreign Relations, and Government Affairs.

In the last Congress, when Republicans controlled the Senate by 55 to 45, they gave themselves a two-vote margin on 10 of the 12 major committees. But the GOP lost two seats in the last election, and committee ratios are being juggled accordingly, sources said.

The new ratios also are part of an effort to enforce a Senate rule restricting each senator to two major committees and one of the seven minor committees. The rule was designed to make the Senate function more smoothly by limiting the workload of senators.

However in the last Congress, 31 senators -- 17 Republicans and 14 Democrats -- served on more than two major committees. The agreement being worked out by Dole and Byrd would exempt about 15 senators -- nine Republicans and five or six Democrats -- from the rule, sources said.

According to sources, the Democrats tentatively have agreed to allow Sens. Alan J. Dixon (D-Ill.), Christopher J. Dodd (D-Conn.), Patrick J. Leahy (D-Vt.), Edward M. Kennedy (D-Mass.), Howard M. Metzenbaum (D-Ohio) and possibly one other to serve on more than two major committees.

Among the Republicans tentatively granted exemptions are Sens. Pete V. Domenici (R-N.M.), David F. Durenberger (R-Minn.), Lowell P. Weicker Jr. (R-Conn.) and Strom Thurmond (R-S.C.).

Several committees are shrinking as a result of the decision to reduce the number of senators allowed to sit on three major committees. This has altered the ratio of Democrats to Republicans on some panels, which in turn has lead to further committee adjustments.

"It's like a Rubik's Cube," said a Senate official. "One change affects the whole thing."

Membership of several panels is up in the air, causing frustration.

"We've been holding off appointing the last couple of legislative people," said an aide to freshman Sen. Paul Simon (D-Ill.). "If you hire a judiciary expert and get the Commerce Committee, you've dug yourself a big hole. If it goes on too much longer, it'll cause a problem."