WELL INTO the evening of the second day of the debate on Star Wars, Sen. J. Bennett Johnston (D-La.) gave a speech that was more of an outburst about the program that President Reagan promised would "make nuclear weapons obsolete."

"Is there anybody here who will get up on this floor and say that the president was right, that we have any possibility of making nuclear weapons obsolete?"

He answered his own question: "Of course not. Of course not."

Nobody else did. Nobody could, not even Sam Nunn, the ranking Democrat on the Armed Services Committee and faithful friend of what the president prefers to call the Strategic Defense Initiative. Nunn, the Senate's hawkish rabbi on defense, said he agreed that "the president's definition of this program is a dream world."

Why, then, was Nunn fighting off every move to cut so much as a dime out of the $2.9 billion the Armed Services Committee had decided was needed to further Ronald Reagan's trillion-dollar fantasy. The committee had cut the president's request for $3.7 billion only with Nunn's indispensable concurrence. And so Nunn, while agreeing with almost every critic, voted no on all amendments, vaguely complaining about "micromanagement."

Johnson's poignant finish was the clear expression of the silent majority in the Senate: "We build this whole air castle out of a speech nobody believes. We purchase all the minds in the country in support, and pretty soon, we in the Senate are rushing out chasing the tail of SDI, not knowing where we are going, but ending up with a country that is closer to bankruptcy than it is now -- and certainly no more secure than it is now."

But Johnston, not an hour before, had voted against his own views as presented in an amendment offered by John Kerry, a Democratic freshman from Massachusetts, who proposed to keep Star Wars at its present budget of $1.9 billion. Kerry made a speech that one senator who disagreed with him called "well thought out and well delivered." He spoke some simple truths, namely that it is not possible to have Star Wars and arms control, that Star Wars would surely violate the ABM treaty and in time, the test ban treaty.

"I told John I probably should have voted with him," Johnston said, revealing much about senatorial thought processes. "But I just hate to be thought of as a foe of all weapons systems. I've been against the B-1 and the MX. And I knew he was going to lose anyway and I didn't want to waste some of my credibility on a lost cause."

Kerry was swamped, 68-21.

Johnston voted for subsequent, less drastic cuts, all of which were decisively rejected. Even a modest attempt by Sen. Albert Gore (D-Tenn.) to limit research to ventures that would not violate the ABM treaty was defeated.

Gore, who as a House member outraged liberal colleagues by his support of MX, said that the American people had no idea of what the program is all about.

"The image of the American people is of a leak-proof population shield. If people understood that this was a defense of our weapons, they would not support it."

Gore said he didn't think Americans wanted to "abandon the ABM treaty, just before a roll call which showed unmmistakably that the Senate doesn't care one way or the other.

To Kerry's argument that the increased funds would fuel research that would "gut the ABM treaty," Sen. John Warner (R-Va.) replied that a decrease would "gut the Geneva arms talks," and would "cut our negotiators off at the knees." Suddenly, he made SDI the reason the Soviets went back to the table, a new theory, but Warner, like other enthusiasts, had no weapons.

Kerry's contention that the Star Wars forces already have more money than they can spend was not contested. He told of recruiters skulking the halls of MIT "like vultures," looking for blue ribbon eastern scientists to give a little gloss to a program that was hatched in the West in the febrile brain of Dr. Edward Teller, father of the H-bomb and favorite of the president.

Nunn chimed in. He had noticed that "there are grants being made all over and we are not quite sure what they are about."

Johnston could not explain what had seized the Senate, why it went over the side for Star Wars, hang the deficits, hang the future of the few treaties in existence.

"There are some true believers, and a lot of people don't question."

That's how it goes in the "world's greatest deliberative body" which has just given the world the most expensive and alarming weapons system since they decided to put multiple warheads on missiles, another step taken with just as much understanding of what they were about.