Former U.S. senator Joseph D. Tydings looked out at the audience of about 75 Montgomery County political activists who sat sweltering in the auditorium of Walter Johnson High School in Bethesda last night and shook his head in amazement.

"Only in Montgomery County could we draw this many people during the dog days in a nonelection year for a forum where no candidates are speaking," Tydings said with a laugh.

The occasion that brought the citizens to the nonair-conditioned auditorium last night was a forum that was supposed to feature representatives of three of the six major Democratic candidates for president. Tydings was there to represent Sen. Gary Hart (D-Colo.). Sen. John Glenn (D-Ohio) sent two members of his campaign staff, Anne Harrison Clark and Carl Ford.

No representative turned up for the third candidate, former Florida governor Reubin Askew. "We'll have someone at the next one," said the man handing out Askew buttons and pamphlets, a reference to the second of the forums scheduled for June 27 when former vice president Walter Mondale, Sen. Ernest F. Hollings (D-S.C.) and Sen. Alan Cranston (D-Calif.) are supposed to be represented.

On occasions such as last night's with the candidates nowhere in sight and the crowds tiny, in some cases the speakers answer questions by either saying they don't know their candidates' position on the issue or don't feel they should speak for them on the issue.

At the end of the 75-minute program, Tydings and Clark were asked how they would differentiate their candidates from one another and what was it about them that qualified them as the most likely to defeat President Reagan next fall.

After Clark had answered that Glenn's record as a moderate and the fact that he would appeal to voters west of the Mississippi made him the most likely to defeat Reagan, Tydings replied by saying, "I would echo Mrs. Clark's statement right up to the name of the candidate and then substitute Gary Hart."

There were no sharp exchanges and no real disagreements. Yet, when it was over, both Tydings and Clark insisted that nights such as these were worthwhile.

"You get a chance to know what kinds of concerns the constituency has," Clark said. "We need to know what the issues are out here. And, we need to let people know that we're interested in them."

Tydings was more practical. "If you are invited to something like this and don't send a representative, it looks like you don't have much of an organization," he said. "I thought Rep. Michael Barnes a Mondale supporter was going to be here tonight. Gary couldn't go unrepresented."

And, both pointed out, if there is fertile ground to find eager volunteers, it is here in Montgomery County where, as one politician once put it, "politics takes the place of sex."

The eagerness of this audience was evident at the end. Even with the oppressive humidity in the room, there were still hands waving when questioning was cut off.

As members of the audience left, sign-up sheets for volunteers awaited them as did buttons--for Glenn and Askew. There were no Hart buttons around. "Issue papers first, buttons later," said one Hart supporter with a smile.