House Speaker Thomas P. (Tip) O'Neo Jr. said yesterday that members of Congress would have rejected their new $12,900 annual pay increases if they had been forced to vote for it.

The 28.9 per cent raise for House and Senate members - and pay increases ranging from 7.1 per cent to 47.6 per cent for federal judges and other high-ranking government officials - automatically took effect yesterday because neither chamber moved to veto it.

Congress had 30 days to act on the pay proposals submitted by former President Ford on the recommendations of the Quadrennial Commission on Executive, Legislative and Judicial Salaries. A move to block the raises was tabled in the Senate and remained locked in committee in the House.

O'Neill said yesterday that the increases "would have been defeated overwhelmingly" if they had been brough to the floor for a vote. He said his collegues would have voted against the measure - even though most of them believe they deserve a raise - because they think the public opposes a pay increase for top federal officials.

O'Neill appeared yesterday on the interview program "Face the Nation" (CBS, WTOP).

The Speaker defended the congressional pay raise. He said members of Congress had received only a 5 per cent increase since 1969. Salaries for news media employees, blue collar workers and civil servants have gone up as much as 90 per cent since then, he contended.

He added: "If you are going to continue to deny the congressman a pay raise, then you're going to have nothing but the rich running for Congress and representing the people of America."

O'Neill said he saw nothing wrong with the way the pay proposals were allowed to take effect.

"The committee [Quadrennial Commission] came back with these recommedations . . . If the committee had come back with a recommedation of no pay raise whatsoever, the the Congress would not have received a pay raise. We're responding to the will of the President's commission," he said.

O'Neill said the commission was created in 1969 to help eliminate political influence on decisions about federal executive salaries.

On other matters, O'Neill said there was "no question" in his mind that the House would approve ethics legislation that has been proposed by a House commission headed by Rep. David R. Obey (D-Wis.).

The commission, officially called the House Commission on Administrative Review, has drafted ethics proposals covering matters ranging from financial disclosure to outside income restrictions for House members.

"Theyve done a magnificent job," O'Neill said of the commission's work. "Their ethics proposals will be before the [House] Rules Committee next Thursday and on the floor the week after.

"The Obey report is an excellent one and I'm going to support it all the way. Ninety-five per cent of it will be passed."

The Speaker also said there was "no question" that Congress will give President Carter authority to reorganize the federal bureaucracy, even though the President has offered no specific proposals for accomplishing that goal.

"He's going to get it exactly as he wants it," O'Neill said. "There's no question in my mind that we're going to pass it [authorize reorganization]. I hope that we can have this out of the way sometime in March.

O'Neill also announced that he and Rep. Jim Wright (D-Tex.), the new House majority leader, will begin looking today into the troubled House Assassinations Committee.

The committee has been racked with dissension - largely caused by the efforts of the panel's chairman, Rep. Henry B. Gonzalez (D-Tex.), to fire committee chief counsel Richard A. Sprague.

O'Neill repeated his claim that he does not have the power to remove Gonzalez, whose actions have been opposed by the majority of the committee's members. He also said he does not believe Gonzalez should be forced to step down.

"We're going to sit down and talk with him [Gonzalez]," O'Neill said. "There's justification to continue this committee . . . We'll iron this problem out."