The congressional drive to slash the Strategic Defense Initiative is motivated by confusion about the goals of President Reagan's "Star Wars" program and pressure to cut defense spending as a result of Pentagon scandals and the Gramm-Rudman-Hollings deficit-reduction act, according to sources on Capitol Hill.

The confusion in part reflects differences in the administration over the intent of the research program, disagreements reflected in recent public statements by key presidential advisers.

Assistant Secretary of Defense Richard N. Perle this week is quoted as saying that the initial purpose of SDI would be to provide defense for the nation's retaliatory force of missiles, rather than the broader goal voiced by Reagan of protecting cities. Perle's view is privately supported by many Reagan administration officials, including some in the Pentagon's SDI organization.

However, after Perle voiced that sentiment recently at a conference sponsored by Time magazine, Paul H. Nitze, Reagan's special adviser on arms control, said, "Maybe it's Perle's view, but I can't see any rationale for it."

Defense Secretary Caspar W. Weinberger yesterday told the Senate Appropriations defense subcommittee that SDI is intended to protect the entire United States and that "the purpose is to have a system that will destroy their missiles before they get near any target."

"I am confident we can do it," Weinberger said in describing SDI as a virtually leakproof umbrella for the United States. He said that such a shield might offer "incidental help" to land-based missiles in the United States.

Another problem for SDI on Capitol Hill stems from befuddlement over when or if it will violate the 1972 Anti-Ballistic Missile (ABM) Treaty. The SDI director, Air Force Lt. Gen. James A. Abrahamson, has repeatedly told legislators that the current test program, which would permit a decision in the early 1990s on whether to proceed with development, would be carried out within the ABM Treaty limits.

At the Time conference, however, Abrahamson said his test program might come into conflict "in terms of the narrow interpretation of the treaty somewhere in 1989."

The first indication of how substantial the opposition to SDI had become on the Hill came in a letter that senators flocked to sign, which called for restraining spending on the missile defense program in fiscal 1987 to 3 percent above the current level of about $3 billion.

Sen. William Proxmire (D-Wis.) drafted the letter, which picked up more than the expected 25 signatures. Then, Sen. J. Bennett Johnston (D-La.), who has some designs on the post of Senate Democratic leader now held by Sen. Robert C. Byrd (D-W.Va.), began circulating the letter on the floor.

The number of signatures has reached 48, with the addition of conservative Sen. Orrin G. Hatch (R-Utah), and Johnston said yesterday he believed he will get three more signatures, giving him a majority of the Senate.

Johnston said that almost all of his colleagues believe the Reagan goal of making nuclear weapons impotent is "unrealistic, yet the Senate continues to pour money into it," a situation he attributed to members' fear of being tagged "anti-defense, anti-technology and anti-Reagan."

He said he would not reject out-of-hand the figure adopted by the Armed Services Committee but said the 3 percent growth figure, or a total budget figure next year of $3.1 billion, should be the goal. He added that a majority of the Senate Appropriations Committee, which will make its voice heard later in the process, had signed the letter.

While the armed services committees were voting, a petition from 1,600 scientists, including many who do weapons work at key government and defense industry laboratories, was delivered to Capitol Hill calling the SDI program wasteful and a threat to expand the arms race.

A competing group of scientists favoring the "Star Wars" program set up shop outside the Senate hearing room where the petitioners held their news conference. Lowell Wood, a physicist at Lawrence Livermore National Laboratory who is a participant in and a a strong supporter of SDI programs, said the anti-SDI scientists signing the petition represent less than 1 percent of the nation's more than 1 million scientists and engineers.