Prince George's County voters have delivered what is potentially the final blow to a well-controlled and cohesive countywide Democratic Party by approving a measure that will reduce the size of the County Council and require all its members to run from separate districts starting with the 1982 elections.

Tuesday's overwhelming vote to amend the County Charter stunned members of the current, all-Democratic council and other Democratic politicians, who in recent weeks worked to defeat the amendment. It guaranteed a major shuffling of power and posts that is certain to leave the already-wobbly party structure splintered and quite likely unable to continue the practice of forging countywide slates of candidates that in the past gave the party its nearly total sweep of county elective positions.

"In political terms, it's a new county," said Del. Timothy F. Maloney, who headed the drive to change the council. Maloney is a maverick Democrat who defeated a member of the party slate in the 1978 primary. "It's a whole new ballgame now. You're gonna see the retirement of at least three members of the council."

Said another Democrat, who has regularly been listed on the party slates: "There's no glue left to hold a countywide ticket together. The tendency will be for the election to degenerate into separate campaigns."

Since county voters approved a home rule charter in 1970, creating 11 countywide (at large) council seats in the process, those posts, along with the county executive seat and to a lesser extent the elected positions in the county courthouse, have formed the basis of the Democratic election day effort.

Prince George's has a 3-to-1 Democratic registration and in county elections, where names are rarely remembered, voters have tended to support the party ticket in both the primary and general elections -- a crucial factor in explaining the dominance of the party organization and the leaders who form the party slate. It also explains how in past years the Democratic leadership, which made the decisions of who would appear on the Democratic slate, was able to ensure party unity and loyalty.

Now, with the passage of referendum question K, all that will change. With the council reduced from 11 to 9 members and its members elected from separate districts, there will be decidedly fewer countywide offices with which to form a slate. That was one goal that the amendment's initiators -- Republicans and Democrats who ran against the party slate in 1978 -- intended. The supporters of K also said they designed the amendment to make the council more responsive to the electorate. Its opponents charged that the amendment will cause parochialism and an emphasis on district issues at the expense of the county.

Aside from governmental considerations, the amendment will force large changes from past tactics in the Democratic effort in 1982. According to most county politicians, the next county elections will be an exercise in free-for-all politics with scores of mini-slates that encompass only legislative districts. While there is speculation that some larger tickets -- possibly a county executive-county council coalition or a county courthouse slate -- will exist, no one believes that in 1982 there will be another coordinated ticket.

The success of question K (it was approved 64,719 to 56,354) and the resounding defeat of two-other council-initiated amendments that would have preserved some countywide council seats also stands as the most serious indication yet of the dissolution of the county's so-called "Democratic machine."

That process, attended recently by much hostility and suspicion, has been under way since 1978 when the Democrats' top two leaders, former County Executive Winfield M. Kelly Jr. and lieutenant governor candidate Steny Hoyer, lost their bids for office.

Since then, the Democratic efforts to reestablish a unified bloc have failed, in part because of defensiveness about the old charges of "machine politics." The party has become disorganized in the state legislature, unable to fend off political assaults by Republican County Executive Lawrence J. Hogan and unable to develop political strategies for the future.

When confronted with question K and its backers, the party mainstream was similarly unable to respond. Initially, it failed to take the ballot question petition drive seriously, then it developed two instead of just one alternative to it and failed to stand united behind one of the alternatives.

Said Maloney: "We didn't beat them, they beat themselves. It was real Keystone Kops. We wanted one thing and pursued it; they didn't know what they wanted."