Recent lopsided votes suggest that President Reagan's Strategic Defense Initiative enjoys solid support in the Senate. But a wide spectrum of senators, including Reagan's Republican point man for the program and the ranking Democrat on the Armed Services Committee, say the fight over SDI, commonly known as "Star Wars," has just begun.

The politics that forced Reagan to cut back his MX ballistic missile program to 40 weapons "very definitely are in the embryo stage" on Reagan's Star Wars program, said Sen. John W. Warner (R-Va.).

Warner, chairman of the Armed Services strategic subcommittee, led the fight on the Senate floor last Tuesday night to keep SDI from being cut back or restructured. His closest call was a 57-to-38 vote against an amendment to reduce SDI funds.

Despite that seemingly comfortable margin, Warner said there is plenty for the White House, Pentagon and pro-SDI forces to worry about to keep the program from being derailed. What saved SDI Tuesday, Warner said, "was a sense of obligation to support our negotiators" at the arms control talks in Geneva.

But SDI backers dare not rely on that leverage, which proved transitory on the MX, Warner said. He agreed with Sen. Sam Nunn (D-Ga.), ranking Democrat on the Armed Services Committee, that Congress had concerns about SDI that were not reflected in the votes against amendments to reduce and reorient it.

Nunn said the administration will not be able to keep Congress and the public sold on SDI unless it comes up with a more defensible definition of what the program is designed to accomplish. The Reagan definition is so broad and so ambitious that an unlimited amount of money could be spent without achieving any specific objective, he said.

Congress and the public eventually will refuse to keep spending billions on a program with no clear goals, Nunn said.

"I don't know a single scientist in the country who agrees with the president's definition of the program," Nunn said.

Reagan called for research into a leak-proof missile defense that would make "nuclear weapons impotent and obsolete."

"What if free people could live secure in the knowledge that their security did not rest upon the threat of instant U.S. retaliation to deter a Soviet attack, that we could intercept and destroy strategic ballistic missiles before they reached our own soil or that of our allies?" Reagan said in his March 23, 1983, speech announcing SDI. "It is reasonable for us to begin this effort. It will take years, probably decades of effort on many fronts. But isn't it worth every investment necessary to free the world from the threat of nuclear war?

"I call upon the scientific community in our country . . . to give us the means of rendering these nuclear weapons impotent and obsolete. I am directing a comprehensive and intensive effort to define a long term research and development program to begin to achieve our ultimate goal of eliminating the threat posed by strategic nuclear missiles."

Sen. J. Bennett Johnston (D-La.), in a speech on the Senate floor last week which stunned some of his colleagues for its bluntness, said nothing has happened technologically in missile defense since Reagan's initial Star Wars announcement although money for the effort has trebled.

"What have we discovered?" Johnston asked. "We have not discovered anything except a speech which was wrong. It was wrong, Mr. President, we know that. Is there anybody in here -- the senator from Virginia, the senator from Georgia, any of you -- who will get up on this floor and say the president was right, that we have any possibility of making nuclear weapons obsolete? Of course not. Of course not.

"We are here doing this little dance tonight," Johnston continued, "because the political landscape has been changed by a speech we all know to be wrong.

"What is the truth of the matter? The truth of the matter is we do not know what Star Wars is. It is a whole collection of technologies that we are going to be chasing out there with the almighty American dollar.

"There is only one thing sure about the ABM [antiballistic missile]," Johnston said, "and that is if we do eventually find it, whatever it is, whether it is a point defense or a space ray or an ionized beam or an electron beam or a neutrino beam, or whether it is is, there is only one thing sure, that is that it is going to violate the ABM treaty and put us into a new space race with the Russians.

"Can it ever make nuclear weapons obsolete?" Johnston asked. "Mr. President, even if you had a leak-proof system, it would not make nuclear weapons obsolete."

No senator rebutted Johnston.

Sen. Barry Goldwater (R-Ariz.), chairman of the Armed Services Committee, said it is difficult to defend SDI because nobody knows what it is going to be. Sen. Jake Garn (R-Utah), an SDI enthusiast, said he did not rise to defend the program because "we knew we had the votes" to defeat the anti-SDI amendments.

Others said the senators do not feel sure enough on SDI to vote their doubts. Sen. William Proxmire (D-Wis.) predicted more anti-SDI votes will be cast in the future as senators learn more about the fuzziness and open-ended nature of the program.

Why did his amendment to reduce the fiscal 1986 authorization for SDI from $2.9 billion to $1.9 billion fail? Proxmire was asked. He cited the concern about jeopardizing the arms control talks, the fact that SDI is still only a research program, Republican unity in supporting Reagan and Nunn's opposition.

"When you have Sam Nunn, you get a lot of Democrats" voting along with him, Proxmire said. Nunn said he opposed the Proxmire amendment because "I made my deal in committee" where Reagan's original $3.7 billion SDI request was cut to $2.9 billion. Nunn said he felt he had a commitment to stick with that figure.

"When you oppose the president of the United States," Proxmire said, "and you oppose the Republican leadership and when you have Nunn, who is viewed by most people in our party as the last word on defense, against you, it's pretty hard to get 38 votes."

Looking ahead, Proxmire said, "the big advantage we have on our side is that SDI, even though we may get into all kinds of unfortunate areas that may fracture the ABM treaty . . . is going to have be pretty much a research program throughout the Reagan administration."

The next president, and perhaps the one after him, will determine whether SDI should advance from paper research to hardware production and deployment, Proxmire said.

"No matter who he is, Jack Kemp or George Bush or a Democrat," Proxmire said, "he's very likely to be opposed" because "it undermines deterrence, makes arms control almost impossible and is a colossal, colossal economic burden and absorbs so many of our scientists who are essential not only in the private sector but in defense areas."

European governments alarmed about the consequences of U.S. deployment of a missile defense are helping the opponents of SDI, Proxmire said. One fear, he said, is that once the United States erected an antimissile umbrella over its territory, the Europeans would be left on their own to cope with the Soviet nuclear threat.

"We're suckers for research," said Sen. David H. Pryor (D-Ark.). "We'll get sweaty palms later when it's time to go from research into development."