Sen. Howard H. Baker Jr. (R-Tenn.) adopted an unexpectedly strong position in opposition to the SALT II treaty yesterday, announcing that it would have to be severely modified to win his support.

At a press conference crowded with reporters and television cameras, the Senate minority leader and would-be candidate for president in 1980 said SALT II "is fatally flawed" because it allows the Soviet Union to maintain its force of 308 blockbuster missiles for which the United States has no equivalent and no defense.

Baker said that he would be willing to work with the Carter administration to make changes in the treaty, but that "if the administration does not signal a willingness to consider amendments, and if the Soviet government does not desist in trying to threaten the Senate, then I will work diligently and, I trust, effectively to defeat this treaty.

Baker's statement was a grave blow to the Carter administration's SALT-selling campaign.The minority leader could play a critical role in the Senate debate on the strategic arms limitation treaty. Administration head-counters always have regarded two-thirds Senate approval without Baker's vote as unlikely at best.

But Baker in his press conference and his staff and associates later emphasized that a door remained open to future bargaining and to Baker's eventual support for SALT II. Administration officials also expressed hope that Baker's statements yesterday were not his last words on the subject.

In tactical terms, Baker did leave room for future maneuvering between him and the administration. But the substantive positions he took were far from what the administration now seems likely to regard as acceptable.

For example, Baker suggested that the best solution to the problems he saw in the SALT II treaty would be for the Soviets to dismantle their 308 blockbuster "heavy" missiles. These are a central element of Soviet deterrent forces, and a demand to dismantle them would challenge the basis of the negotiated SALT II agreement.

In March 1977, President Carter proposed that the Soviet reduce their heavy missile force to 150. This idea was rejected and ridiculed by the Soviet side.

Baker did not suggest yesterday what the United States might give up in return for new Soviet concessions.

The 308 heavy missiles were permitted to the Soviets in the SALT I treaty negotiated by President Nixon, and again in the Vladivostok accord signed by President Ford.

Baker voted for SALT I and endorsed the Vladivostok agreement.

Baker said yesterday that SALT II "provides a substantial strategic superiority to the Soviet Union." Even more important, he said, was "the signal it sends" that the United States is "willing to accept . . . a questionable American strategic adequacy born of a decade of vast Soviet military growth coupled with a gradual diminution of American strength."

Apart from the problem of the 308 heavy missiles, Baker said an amendment to SALT II is needed to improve U.S. abilities to monitor Soviet compliance with the pact. He implied a desire to bar any encoding of test information radioed to earth by rockets in flight, and to require that both sides notify the other before any test vehicle is fired.

Baker also reiterated his belief that the Soviet medium-to-long-range bomber known as Backfire should be counted under SALT's overall limits on strategic arms. The White House regards this as a "killer" amendment that would scuttle SALT II.

U.S. officials with the president in Tokyo for an economic summit conference did not seem deeply upset by the Baker statement. But they had no immediate reaction, saying they would wait to see a complete version of Baker's remarks.

Aides to Baker suggested that his strong statement yesterday was provoked largely by Andrei Gromyko, the Soviet foreign minister.Monday Gromyko warned the Senate against making any changes in SALT II, threatening dire consequences if changes were made.

Said Baker yesterday: "I respect the Soviet Union, I hear and understand Foreign Minister Gromyko, and my reply is that the Senate will work its will and the American Republic will survive and prosper without that advice from Russia."

Baker's decision to come out publicly with strong and explicit criticisms of SALT II followed his meeting last week with President Carter, when the two discussed the possibility of amendments. Baker had previously made clear his desire to serve as a broker between the White House and moderate senators who wanted to support SALT but also wanted to alter it.

Carter gave Baker no direct encouragement at that meeting, which led the Tennessean to conclude that he ought to speak out. He consulted colleagues and advisers, and used a hardline expert from Georgetown University, Edward N. Luttwak, to help prepare his formal position.

At his press conference yesterday Baker was asked what sort of world he would inherit in 1981 if he is elected president next year after SALT II is defeated. He replied that his "second priority" as president would be to negotiate a new SALT pact.

The first priority, he said, would be to build up the country's defenses by producing a new bomber, adding to the Navy and strengthening conventional forces. (All those steps would be permitted under the SALT II treaty.)

Baker minimized the risk that rejection of SALT II would end the arms control process and totally alienate the Soviets. "The process is too important for that, it is vital," Baker said, expressing hope that the two countries would return to the bargaining table "within hours" if SALT II is rejected.

Baker also expressed confidence that the NATO allies would understand a Senate vote against SALT II and would not react too negatively.

Baker vigorously defended the Senate's - and his - right to change a treaty negotiated by the president. "I have a duty to advise on the formulation of the most important treaty this nation has undertaken since World War I" he said.

It is now up to President Carter and the Soviets to indicate some willingness to alter SALT II, Baker said. If they do, "I would be anxious" to resume efforts to find "reasonable amendments" that would make the treaty acceptable to him and 66 other senators.