The Senate adopted strong language governing future interpretations of the U.S.-Soviet medium-range nuclear missile treaty as it headed toward final approval of the pact Friday.

"Today is T-Day," said Majority Leader Robert Byrd, D-W.Va., opening what was expected to be the last day in two weeks of Senate consideration of the treaty. "I think we've closed all the loopholes we know about in this treaty, and it's a much better treaty than when it was sent" to Congress.Byrd said he hoped to have Republican help in beating back "troublesome" amendments, and complete the pact in time to get it to President Reagan for his weekend summit with Soviet leader Mikhail S. Gorbachev.

Thursday's session had ended on a testy partisan note, with Byrd threatening to let the treaty languish if Republicans continued to seek what he saw as debilitating amendments.

Approval of the treaty was considered certain because it requires a two-thirds vote of the Senate, or 67 votes, and as many as 90 senators have expressed support for it. But the possibility of delay remained alive.

Byrd warned GOP senators that they risked embarrassing their own president by failing to deliver the approved treaty in time for the Sunday start of the summit.

"This is no empty threat," Byrd admonished senators. "If we're going to continue to have Mickey Mouse amendments like this, the president is not going to have his treaty before he leaves the summit."

White House chief of staff Howard Baker was standing by to carry the ratification papers to Reagan, who was resting in Helsinki, Finland, en route to Moscow.