The Senate ought to vote "expediously" to end the mutual defense treaty with Taiwan, and at the same time should outline its role in terminating future treaties, Senate Leader Robert C. Byrd (D-W. Va.), said yesterday.

He told reporters that he strongly disagrees with a federal judge's ruling last week that President Carter acted illegally when he unilaterally abrogated the treaty as of next Jan. 1. U.S. District Court Judge Oliver Gasch's decision, Byrd said, "leaves a very important foreign policy decision up in the air."

Gasch ruled that abrogation would require a two-thirds majority vote in the Senate or a simple majority vote in both the Senate and the House. "I don't know what his rationale is for that," Byrd said.

Byrd argued that, while the Constitution is silent on the subject of abrogating treaties, the Senate assigned itself no role when it gave either party to the Taiwan pact the power to back out with a year's notice.

"Legally, I don't think the Senate needs to vote again at all, " Byrd said, "but it would be useful so there would be no doubt there is a clear majority in the Senate" for an end to the treaty with Taiwan.

Such a vote would require only a majority and not a two-thirds agreement, Byrd said, because the Constitution clearly spells out occasions for the larger requirement.

He said he is negotiating with Sens. Barry M. Goldwater (R-Ariz.) and Harry F. Byrd Jr. (Ind.-Va.) who filed the anti-treaty suit that led to the judge's ruling in order to come up with a "vehicle" for Senate debate.

Byrd said it would have two parts: one endorsing the new position on Taiwan and another establishing a role for the Senate in ending any future treaty. The exact terms of that role are not decided, Byrd said. He added that there is "a good majority" in the Senate for withdrawal from the Taiwan agreement.

The Goldwater-Byrd suit against the move was backed by 23 other conservative lawmakers. A senate majority passed a resolution in June that the Senate should have been consulted on Carter's action, and Gasch's ruling last week was based on that action.

Byrd said he thinks the pending trade agreement with the People's Reublic of China will not be affected by the flap over the Taiwan situation. The trade pact is not a treaty and probably will not come up until after the Taiwan case is settled, he said.

Turning to the Strategic Arms Limitation Treaty (SALT) debate, Byrd said he is concerned that some Republican presidential hopefuls "are making a partisan political issue out of it."

He declined to name names. The Senate debate, once it begins, should not take longer than three weeks, and might be televised nationwide if the senators agree to a time limit, Byrd said.