Gov. Harry Hughes will take the politically explosive step of proposing a final redistricting plan that helps Prince George's County hold onto most of its power in the General Assembly, a move expected to alienate at least one key legislative leader, sources said.

The governor's decision could enable Prince George's to retain all eight of its Senate seats for the next decade, delighting the county's General Assembly delegation, which had threatened to oppose Hughes on key issues if he failed to redraw its political boundaries.

"We won big. We got it all," said a beaming Prince George's legislator who declined to be named.

The Hughes proposal, expected to be delivered when the session opens Wednesday, rejects the recommendation of his own five-person advisory commission, which included House Speaker Benjamin L. Cardin and Senate President James Clark. It also comes as a personal blow to Clark, whose home base--Howard County--was to gain an extra legislative district under the commission plan.

"I'm very unhappy," said Clark, who was told of the governor's decision today. "They practically decimated me."

The commission had called on Hughes to reduce the size of the formidable Prince George's delegation, shifting one of its eight Senate seats to a district dominated by the boom town of Columbia in Clark's Howard County.

Hughes, according to sources, decided instead to split Columbia between two districts, giving Prince George's a chance to dominate one of them. The governor's proposal means that Prince George's, which hasn't grown as fast as the rest of the state, would have a chance to elect almost as many legislators in the 1980s as in the 1970s.

The Hughes proposal follows most of the commission's plan for other areas of the state. Since the commission plan had wide support in both houses, the Hughes proposal is expected to pass easily.

Although Hughes was expected to make some changes in the Prince George's and Howard boundaries, few politicians predicted that he would risk displeasing Clark, whose support is considered valuable. In addition to splitting Columbia, the Hughes plan slices off a piece of northwest Howard County that Clark had hoped to retain, shifting it to a Western Maryland district.

"I'm not one to overstate my reactions, but let's put it this way: I'm disappointed," Clark said. "I'm not disappointed for myself personally, because I can survive. (But) a lot of Columbia people are going to make a lot of noise."

Hughes said earlier this week that splitting Columbia would give the outer suburban town a chance to dominate two districts if it continues to grow rapidly through the 1980s, as is expected. Clark did not see it that way, suggesting that Hughes was attempting to appease populous Prince George's County. "It's a question of whose ox is getting gored," he said.

Prince George's legislators had argued that Hughes would be politically wise to help their delegation since the county's strong Democratic organization could help him in this year's election. But the county's House delegation chairman, Robert Redding, said today, "I've been in meetings with the governor and neither he nor I ever mentioned any quid pro quo."

Hughes' plan hurts the political prospects of one commission member, Columbia's Vernon Gray, who was expected to run for the House of Delegates and would have benefitted from the commission's plan. Hughes was said to be "very upset" when he learned of Gray's aspirations which threatens to color the plan's neutrality.

The Hughes proposal would create a new district shared by the eastern half of Columbia and the Laurel area of Prince George's. Laurel and West Laurel would constitute more than one-third of the new district. As a result, Prince George's would have a chance to elect two delegates and the senator in the new district, rather than only one delegate, as the commission proposed