Early Senate ratification of the U.S.-Soviet treaty eliminating Intermediate-Range Nuclear Forces (INF) was threatened last night by a sharp split between the Reagan administration and Senate Democrats over the Senate's constitutional role in approving treaties.

The bitter dispute broke into the open last night when Senate Majority Leader Robert C. Byrd (D-W.Va.) and Sen. Sam Nunn (D-Ga.) sent an angry letter to Secretary of State George P. Shultz threatening to delay a vote on the treaty unless the administration changed its position on the constitutional issue.

The abrupt confrontation came even as the Senate Foreign Relations Committee's scheduled month of INF ratification hearings reached the end of the second week with most witnesses calling for approval of the accord, signed by President Reagan and Soviet leader Mikhail Gorbachev at the Washington summit in December.

The controversy, cloaked in legalisms but reflecting a smoldering arms-control dispute between the administration and congressional Democrats, centers on whether sworn Senate testimony by administration officials about arms-control treaties is "authoritative," and therefore legally binding in interpreting the scope of treaty provisions.

Since 1985, the administration has contended that its officials' treaty testimony might not reflect the true meaning of an arms-control accord.

But in behind-the-scenes debates in recent weeks, Nunn and Byrd have sought to use the administration's desire for quick approval of the INF Treaty to obtain Shultz's repudiation of this administration position.

Senior Reagan administration officials expressed this view while defending a "broad" or permissive reading of the 1972 Antiballistic Missile (ABM) Treaty that conflicted with Senate testimony by key officials in the Nixon administration at the time. The contoversial "broad" ABM interpretation, adopted by President Reagan in 1985 to justify elaborate space tests of "Star Wars" missile defense technologies, has since been rejected by a majority of both houses in Congress.

Nunn and Byrd have negotiated for weeks with senior administration officials, including Shultz, on the text of a Shultz letter to Congress about the INF Treaty that could have undermined a "broad" reading of the ABM Treaty by conceding that executive branch statements during treaty ratification hearings are "authoritative" and legally binding.

Congressional aides and administration officials said Shultz agreed on such a statement in a secret Monday meeting with Byrd and Nunn, but changed his mind after discussions Wednesday evening with a group of angry conservative Republican senators who support the "broad" reading of the ABM Treaty so that the administration can pursue more elaborate "Star Wars" space tests. Senate Democrats became aware of Shultz's reversal late yesterday afternoon when Shultz wrote Byrd a 1 1/2-page letter on the INF Treaty hearings without promising that the administration's testimony on the meaning of the treaty was legally binding.

Shultz's letter, which effectively broke off his negotiations with the Senate Democrats, accompanied roughly 30 volumes of classified records from the INF negotiations that the administration turned over to the Senate. Shultz said that if they wished, the senators and their staff could read through the voluminous documents to determine the basis for the administration's interpretation of the INF Treaty.

Nunn and Byrd subsequently criticized Shultz's apparent reversal in a letter delivered to the State Department late last night, charging it would lead to "inevitable delay" in the Senate's approval of the INF treaty.

They said Shultz's position meant it was simply "too bad" if administration officials "misinformed" the Senate about the meaning of the INF Treaty. "Unless modified, {the position} puts the Senate on notice that if it relies on the testimony of the administration . . . it will do so at its own risk."

The legislators also said Shultz's position left them with little choice but to review the INF negotiating record "exhaustively" and determine if administration officials had testified correctly about the treaty's meaning. Nunn and Byrd also threatened to write congressional "understandings, reservations and amendments addressing every point of the interpretation."

They threatened that such an approach "could undermine the treaty ratification process." They added that until Shultz appeared before the Senate Armed Services Committee, chaired by Nunn, "to resolve this impasse," the committee would bar any further testimony on the INF Treaty by other administration witnesses.

The Senate Foreign Relations Committee has formal responsibility for treaty ratification, but the Armed Services Committee typically plays a key supporting role.

A State Department official said last night, however, that there would probably be a "divergence of views" between Nunn and Shultz at such a hearing.

The official confirmed congressional reports that, before the Wednesday meeting with Senate Republicans, Shultz and Nunn "were pretty far along" in agreeing on the text of a letter meeting Nunn's concerns.

Several critics of Shultz's position said last night it would generate additional support for a treaty "reservation" proposed by Sen. Joseph R. Biden Jr. (D-Del.) barring implementation of the INF Treaty without agreement that the Constitution blocks the executive branch from adopting a treaty interpretation different than what it tells the Senate during ratification hearings.