Four days after President Reagan and Soviet Leader Mikhail Gorbachev signed a landmark nuclear arms-reduction treaty at a December White House ceremony, Secretary of State George Shultz predicted that the United States Senate would easily vote to ratify the accord. ``We believe the treaty will sell itself,'' proclaimed Mr. Shultz. Administration officials don't say things like that today. Senate approval of the Intermediate Range Nuclear Forces (INF) Treaty does not look as easy as it did in December, when dozens of legislators hurried before television cameras to endorse the pact. Instead, it has become bogged down by an array of nettlesome controversies concerning such basic matters as the scope of the Senate's treaty-approval powers and the meaning of the word ``weapon.''
Virtually no legislator or administration official says the treaty's eventual ratification is in danger. But many of them are concerned that questions will not be resolved before President Reagan's trip to Moscow at the end of the month. That would leave him and Mr. Gorbachev without a major agreement to showcase during their superpower summit.
Senate Democratic leaders have said they will open floor debate on the treaty May 11 - nearly six weeks after the accord was overwhelmingly approved by the Senate Foreign Relations Committee - provided that four ``important loose ends'' have been tied down. But those Democrats, all of whom are among the treaty's strongest supporters, offered only cautious assessments of the prospects for ratification before the Moscow summit.
Moreover, they and their colleagues worry that the difficulties that have arisen so far may mean that the Senate will be unable to take up the treaty as at present scheduled.
One of the loose ends they cited apparently will require further US-Soviet negotiations. It involves a recent dispute over comprehensiveness of verification procedures for the pact's proposed ban on medium and shorter-range missiles in Europe.
Another dispute centers on the question of whether the US can inspect missile containers at designated Soviet weapons facilities that contain single missile stages, as the US claims, or complete missiles, as the Soviets maintain.
Still another disagreement concerns a Soviet attempt to declare off-limits to US inspectors certain areas in three of the USSR's 133 facilities covered by the inspection agreement. In both cases, administration officials contend that the Soviets are backpedaling from some previously negotiated requirements for on-site monitoring of compliance with the weapons ban.
White House officials assert that the dispute can be resolved without difficulty. They urge the Senate to move ahead with its consideration of the treaty. But at a press conference Friday of the Democratic leaders, Senate Armed Services Committee chairman Sam Nunn (D) of Georgia said he was only ``cautiously optimistic'' that the matter would be so easily settled.
``I believe the problems can be reconciled, but I believe the Soviets should understand that this is not a good way to start out - by challenging some clear provisions of the treaty,'' he said. ``It is enormously important that we go into this treaty with good faith on both sides.''
In addition to the monitoring dispute, Senate Intelligence Committee chairman David Boren (D) of Oklahoma said that the full chamber would be unable to take up the treaty until the Soviet Union clarified, in writing, that the treaty's ban on all ground-based missiles with ranges of 300 to 3,400 miles also applied to futuristic versions of those missiles.
In addition, Senator Boren said, Soviet and US officials would have to agree on a formal definition of the term ``weapon'' as used in the text of the treaty.
Finally, Boren said, Senate consideration of the treaty would be predicated on a clarification by US officials of the verification techniques for such a ban on futuristic weapons, as well as a commitment from the administration to upgrade the satellite surveillance systems that will be used to help monitor the accord.
A senior Democratic Senate staff aide said that the administration appeared confident it could satisfy ``two or three'' of those four items. But the aide said the administration was balking at the demand for a commitment to new, high-technology surveillance equipment.
At the same time, the aide pointed out, a partisan disagreement over the ability of an administration to ``reinterpret'' a treaty without Senate consent was likely to trigger rancorous debate on the Senate floor. Until those two items are resolved, the aide asserted, the treaty will not win Senate approval.