IF A PUBLIC DISPLAY of tedium were an actionable offense, someone would have called the cops to close down the proceedings in the Senate Caucus Room last Tuesday night when there curred something that was billed as a debate between Republicans and Democrats on defense policy.

About three-quarters of the way through what turned out to be a meeting of friends of the Pentagon, moderator Ted Koppel said, a little desperately, "Well, let me continue to operate under the premise that a debate occasionally brings about some disagreement between the two teams . . ."

Commentator Bill Moyers sternly lectured the networks and the public for their apathy toward the debate. Both groups are, as a matter of fact, to be congratulated. It is too much to ask people to fight sleep so they can learn there is not a dime's worth of difference between the parties when it comes to guns and butter.

A realistic view comes from a Democratic Party official who was one of the architects of the disaster. "Off the record, the only good thing about it was so that so few watched it."

The sponsors, the League of Women Voters, the good people who brought us some of 1980's more memorable moments -- "There you go again" and little Amy's nuclear angst -- cannot be blamed. Fair to a fault, as always, they let the parties decide who should speak for them. It is not their fault that the Democrats chose Majority Leader Jim Wright to "me-too" one of the preeminent hawks of the Senate, Republican John Tower of Texas.

Says Dorothy Ridings, league President, "None of us was really pleased at the way it turned out."

Sam Nunn (D-Ga.) was perhaps the inevitable Senate spokesman, given the conservative defense views of Democratic leader Robert C. Byrd. Nunn is one of those southern Democrats who dazzle their colleagues and baffle the public with their command of Pentagonese. He sounded like a member of the joint chiefs with his references to "21/2 wars." That kind of talk makes some Democrats feel secure.

The choice of Wright, who is not a member of the Armed Services Committee, tells you much more about the Democrats. In fact, it tells you that Ronald Reagan's rivals prefer to hide on the dangerous subject of astronomical military spending -- even though it is tied to the economy, the issue they expect to ride to victory.

One embarrassed Democrat, who did not wish to be identified, confided, "We didn't want to take any chances. We didn't want to give the Republicans a chance just 25 days before the election to say we are weak on defense.

Wright, whose approach to the listening audience was almost flirtatious -- he did a great deal of blinking and smiling as if there were some Miss Bluebonnet of Texas just beyond camera range -- took every opportunity to assure the voters that while he mourned the current plight of the deprived, he doesn't blame the defense budget.

So anxious was he to show himself a good soldier in Reagan's army that he did not mention the fact that he voted on the House floor, along with a majority of his party, for the nuclear freeze. Some Democrats, including Leon Panetta of California, who is running against Ronald Reagan's guest-intruder, Gary Arnold, complain that "Wright cut the ground out from under us on the freeze."

Nunn of course, is opposed to the freeze. But at least he spoke up for the victims of the president's recent slanders against its advocates. "I really do not think that is the kind of statement that ought to be made . . ."

It is possible that the conspiracy to bore people to death was bipartisan. There can be no other explanation for the presence on the program of an obscure Republican congressman from Alabama named Jack Edwards. His words, his ideas, his delivery, were all of the room-emptying variety.

Some inside speculation goes that he was chosen by his intimate friend House Repure are inscriptionyblican leader Robert Michel. Michel knew that Edwards would not deviate from Reagan defense dogma and could also be trusted to get in a word about Republican dissent from the president on the matter of the Soviet pipeline sanctions, which are killing Michel back home in Peoria.

Maybe it was complacency, not cowardice, that drove the Democrats in the their debate choices. They think they will sweep in November, and wish to do nothing to rock the boat. Some more thoughtful Democrats are not so sure.

Sen. Carl Levin (D-Mich.), the only member of the Senate to vote against the defense budget, wonders if the public thinks that any Democrat would do a better job than Reagan.

What the Democrats said on that "debate" was that on outspending the Soviets for weaponry, they would do nothing different. Which is a reason that some serious people think that the election which is making everyone so nervous will be decided by non- voters. Certainly Tuesday night's mishap gave them no reason to go to the polls.