About 10 minutes before the Maryland House of Delegates prepared to vote last week on Gov. Harry Hughes' proposal to broaden the rules for Medicaid-funded abortions, his pro-choice allies on the House floor were taken by surprise.

Del. Lewis R. Riley (D-Wicomico), considered a "sure" pro-choice vote in the General Assembly, informed them he was going to vote against Hughes' measure. Legislators supporting Hughes quickly phoned the governor to prod him into some last-minute lobbying. It was too late.

Eight "probable" votes--seemingly suddenly--had shifted against them. It was all part of a slick strategy by the anti-abortion forces in the House that left both sides somewhat stunned at the outcome. The result was a setback for the governor and the pro-choice movement, which had been confident after last year's election that, with more than 50 new members, the legislature would be more inclined to liberalize abortion funding during this session.

"I was really upset after that vote," said Del. Mary Boergers (D-Montgomery), whose own lobbying efforts had failed. "I got off that floor so fast. If I had seen one of the delegates who switched votes I thought I would have punched them out."

What happened was a combination of misjudgment by the pro-choice group and Hughes and some ingenious wording by their anti-abortion foes.

For months the pro-choice group, led by lobbyists for Planned Parenthood and cheered on by Hughes' wife Pat, had counted votes and plotted strategy. The lobbyists had convinced Hughes that the current restrictions on abortion funding, passed in 1980, were too severe.

Abortions funded by Medicaid state-wide had dropped by 50 percent--from 6,500 in 1980 to 3,200 in fiscal 1982--since the restrictive language was adopted. And health officials determined that, partly as a result of the language, women seeking Medicaid abortions could get them at fewer than five clinics in Maryland.

Hughes agreed that a change was needed but he would not go as far as urged by Planned Parenthood, which asked him to submit language that placed no restrictions on Medicaid abortions. Instead, thinking it was politically feasible, Hughes submitted broader language in his 1984 budget proposal that would enable women to get Medicaid abortions for mental health reasons.

On the eve of last week's vote, the pro-choice group thought it had the votes to win, and on the day of the vote Hughes says he made only "a few calls" to lobby for his measure.

What Hughes and the pro-choice group failed to anticipate was that the anti-abortion forces would blame the governor for trying to subvert a "compromise" worked out in 1980, as well as for "stirring up" the abortion issue again this year.

The tactic turned out to be unbeatable.

It began with Del. Timothy Maloney (D-Prince George's) offering an amendment patterned after the federal Hyde amendment that would virtually eliminate state-funded abortions. It failed by four votes.

Then Del. Paul Weisengoff (D-Baltimore) began the "compromise" campaign in earnest. After lobbying legislators on the floor, he urged his colleagues (who minutes before had rejected the Hyde amendment) to turn down Hughes' proposal also. Instead, he said, "compromise." Compromise meant retain the current restrictive budget language on abortion. Compromise, Weisengoff implied, was the only right thing to do in a legislature that is divided strongly on the issue.

After Weisengoff spoke on the floor, a group of key Eastern Shore "probables" voted for his "compromise" amendment. The pro-choice group was furious.

"To call it a compromise!" Boergers said the next day, still angry at the outcome. "I was real upset."

Steve Rivelis of Planned Parenthood said: "A vote for this and they felt they could still say they were pro-choice. And they could also look good to the otherside.

The hard-core anti-abortion forces were reasonably pleased by their success. "The morning after the November election I never could have imagined this happening," said Maloney. "When I drew up the Hyde amendment I thought we would lose by 10 votes not four . I don't think you'll get the House to vote for a strong pro-life or pro-choice measure for a long time. People want to stay in the middle."

In the daze that followed the voting, one had to wonder what kind of "compromise" the pro-choice legislators could have won if, instead of hoping Hughes' language would be adopted, they had also offered a more extreme amendment to lift all restrictions on abortion.