A legal dispute between Senate Democrats and the Reagan administration over the U.S.-Soviet treaty eliminating intermediate-range nuclear forces (INF) intensified yesterday as key senators proposed to extract even greater concessions from the administration.

Sen. Joseph R. Biden Jr. (Del.), the Foreign Relations Committee's second-ranking Democrat, confirmed plans to attach a formal "condition" to the INF Treaty that would resolve the dispute by affirming the Senate's power to make legally binding testimony given by the executive branch during ratification hearings.

Biden's remarks were echoed by the chairman of the Foreign Relations Committee, Sen. Claiborne Pell (D-R.I.), who said yesterday, through a spokesman, that "the committee plans to put forward a solution which will resolve the constitutional issues without delaying {Senate approval of} the INF Treaty."

Biden, in a statement, said he and some others on the committee would not be satisfied by a proposal by Senate Majority Leader Robert C. Byrd (D-W.Va.) and Armed Services Committee Chairman Sam Nunn (D-Ga.) to resolve the dispute through a written pledge or sworn testimony from Secretary of State George P. Shultz.

Shultz declined last week to provide the assurances sought by Byrd and Nunn that administration statements to the Senate on the INF Treaty were authoritative and legally binding, but he indicated his willingness to do so at a private meeting with them.

Nunn and Byrd brought the dispute into the open last Friday when they threatened to delay approval of the treaty until Shultz changed his mind.

Shultz had decided against making such a pledge under pressure from Republican senators who complained it would undermine the legal basis of the administration's controversial "broad" interpretation of the 1972 Antiballistic Missile (ABM) Treaty.

The administration, in advancing the "broad" interpretation, said it was free to ignore Senate testimony by key executive branch officials at the time who supported a traditional or "narrow" reading of the treaty.

Biden, like Byrd and Nunn, has been highly critical of the "broad" interpretation and of the administration's assertion that Senate testimony by executive branch officials may not reflect a treaty's true meaning.

Biden said yesterday, however, that "no statement the Senate can eke out of the administration through private negotiations {with Nunn and Byrd} would solve the problem. The only satisfactory solution is the condition I intend to attach to the INF Treaty upholding the Senate's constitutional role" by ensuring that executive branch testimony is legally binding.

Biden said "such a condition would have far greater effect than any negotiated language contained in a letter from Secretary Shultz." He said although such language might bind the Reagan administration, it would not bind future administrations or confirm Senate treaty approval powers.

Under Biden's proposal, the administration could implement the INF Treaty only if it agreed to follow "the understanding of the treaty shared by the executive and the Senate at the time of ratification . . . as required by the Constitution."

An aide to Nunn acknowledged that a letter from Shultz probably would only bind the Reagan administration, but said "this administration is the problem, and future administrations would have to accept the political costs of reversing" Shultz's written declaration.

Sen. Arlen Specter (R-Pa.), who pressured Shultz last week not to compromise with Byrd and Nunn, yesterday described the assurances they sought as "purely political concessions . . . of no significance under international law."

In a series of letters, Specter urged Shultz to hold firm and called on Byrd and Nunn to drop their threats to delay the hearings.

Staff writer Helen Dewar contributed to this report.