Richmond M. Keeney, Republican candidate for Montgomery County executive, joined the entire Republican County Council slate yesterday in denouncing incumbent Executive James P. Gleason's proposal to build a large sewage treatment plant in the middle of affluent Potomac.

If elected, the Republicans said they would opposed putting the $62 million plant on the 455-acre Avenel Farm tract. The solution to the country's sewage treatment problems, they said, lies partly in persuading Prince George's County to share its sewage treatment excess capacity that Montgomery County residents helped pay for.

The Republican statement was immediately critized by their Democratic opponents as a "blatantly political" and "desperate" act in an effort to win election on Tuesday.

"If at all possible, we should have a site more remote from the people," said Charles W. Gilchrist, Democratic nominee for county executive, "but there are not enough hours left in this campaign to give it the study it needs. I intend to make a decision based on all the facts."

When Gleason proposed the Potomac site Tuesday in an attempt to resolve a six-year dilemma before he leaves office, the Democratic council incumbents seeking reelection were furious about what they called his "11th-hour demand."

The three Democrats seeking reelection and the four Democratic candidates for County Council seats, have thus for avoided taking an explicit position on the proper site for one or more new sewage treatment facilities that planners estimate the county will need in the next two decades.

"We need to make a studied response" to Gleason's proposal, incumbent Esther Gelman said yesterday.

Gleason asked the council to meet this week to review his highly controversial proposals, but the council member angrily declined and instead scheduled the work session for Nov. 10, three days after the election.

The day of Gleason's proposal "was our last day of business before the election, and I don't think we should be critized for taking a few days off," said Council President Elizabeth Scull, who is seeking reelection.

Scull said that if the two counties cannot resolve their ongoing feud in the bicounty Washington Suburban Sanitary Commission, Gleason's choice is "probably the best . . . It does permit discharge of effluent below water intakes, which has been a crucila problem in all this."