All last night and today, Prince George's County legislative leaders here have been mulling plans to counter the devastating loss yesterday in a crucial vote on their proposed $35 million convention center.

The House Appropriations Committee vote against a $20 million bond issue to finance construction of the center was not their only loss. Many county delegates also feel they lost face, and amid the wounded egos, embarrassment and resentment today, some county legislators were searching for culprits and talking openly about how to "get even."

"There is an Irish saying that says, 'Don't get mad, get even,'" said Del. Gerard F. Devlin (D-Prince George's). "Our credibility is at stake."

"They have a lot of bills they want . . . and they let me down, they beat me," Del. Andrew O. Mothershead (D-Prince George's), spnsor of the convention center bill, muttered last night after the defeat.

In all the conversations, county delegates spoke with particular bitterness about what they regard as their abandonment by Baltimore City delegates on the Appropriations Committee after two years of trying to build a coalition with the powerful city delegation by voting for Baltimore bills.

"For two years I acted as a whip to give Baltimore City votes on the subway, the Orioles loan . . . This one hurts. It's a crusher," said Devlin.

"We're not going to get mad to the point of frothing at the mouth," said Del. John J. Garrity (D-Prince George's). We're just vexed, which is an understatement."

"They just didn't count the votes," said Del. Paul E. Weisengoff (D-Baltimore City) chairman of the city delegation. "I'm embarrassed because the three city votes I committed didn't come through."

Weisengoff agreed with Del. John R. Hargreaves (D-Caroline) House Appropriations Committee chairman, that regardless of any city-council coalition the votes were not in the committee and wouldn't be there this session. Why the votes were not there was a matter occupying a lot of minds today.

The "crusher" was a 16-to-6 defeat of a bill to give $20 million in state bonds for the proposed county convention package. Another $15 million was to come from a private developer to construct the center at Largo next to the Capitol Centre areas.

The appropriations committee has only five Baltimore City and four Prince George's County delegates, not enough to swing the result even if they had voted as a bloc. All four county delegates voted in favor of the bill but only one Baltimore City delegate - Vice Chairman R. Charles Avara - joined them. David L. Scull (D-Montgomery) was the sixth vote.

The tally was examined and re-examined through the day as Prince George's delegates looked for someone on whom to blame their defeat. Gov. Martin Mandel was brought up as the only person who could have caused such a defeat but there was no evidence of that.

Del. Frederick C. Rummage, chairman of the all-Democrat Prince George's delegation, was acting as a booster for his people, giving them hope that through some means the convention center proposal could be approved.

But with the District of Columbia moving fast on its $110 million convention center proposal, the urgency was putting a bite on the good humor of the delegation and they were scurrying.

"The Prince George's delegation has finally discovered the appropriations committee: they've been walking through here all day," said Del. William M. Linton (R-Baltimore County) after the committee had finished its day's work.