The proposed Prince George's County convention center failed its first crucial test today when a legislative committee overwhelmingly rejected the use of state funds for its construction.

The 16-to-16 vote by the House Appropriations Committee against a $20 million bond issue for the center left some county officials speechless late yesterday.

Some had earlier expressed confidence about the chances of success in the legislature and regarded the District's efforts to also build a convention center as the real threat to the country's plans.

"We had the votes but Baltimore City and Baltimore County let us down," said Del. Andrew O. Mothershead (D-Prince George's). "It beats me why they did it. There are many things we can do yet, a lot of important votes coming up."

Mothershead said he would petition the bill to the House floor, a relatively rare practice that allows 15 delegates to bring a bill to the full House over the objections of the committee.

The delegates did not debate the bond issue today before voting. But during previous discussions in and out of committee, legislators had expressed concerns about approving spending so much future tax money on a convention center, especially in a year when a sales tax increase is likely. There were also questions raised about using state money in a joint business venture with a private developer.

The Prince George's proposal called for a $35 million convention center to be financed by the bond issue and $13 million from county developer, William Levitt Jr. Levitt, who controls the lands near the Capital Centre, which would have been the convention center's site, would also have operated the new facility with a $1.5 million annual operating subsidy from the county.

The vote late today was preceded by several days of lobbying by County Executive Winfield M. Kelly Jr. and county political leader, Peter F. O'Malley. The role of O'Malley, who was both the originator of the financing proposal for the center and the attorney for developer Levitt, was also questioned during hearings on the proposal.

There were concerns expressed privately by some delegates today about "pressure" in the form of vote-trading proposals in connection with the prince George's project.

The suggestion that a deal had been made was, in fact, raised on the House floor earlier today by Del. Richard W. Emory Jr. (D-Baltimore) Emory was seeking defeat of a bill favored by most of Baltimore delegates to impose tolls on a tunnel under construction in the Inner Harbor of Baltimore. The proposal won near solid support from the Prince George's County House delegation.

"I hope this chamber is not a marketplace for goods or barter" Emory said afterwards.

In fact, vote trading is common here on major issues, although the participants usually deny there was any "barter." Numerous Prince George's County delegates voted for a Baltimore convention center last year in exchange for Baltimore's support on the county's convention project.

There was also private talk today about Prince George's delegates helping Baltimore legislators block an effort to convert an old can factory in that city into a prison in exchange for city votes for the convention center.

It was this alliance of business and politics that disturbed some of the delegates who voted against the bill as well as the costs to the state.

"I'm honestly disturbed by the private factor, the private involvement," said Del. Frank C. Robey Jr. (D-Baltimore). "What also concerns me, in fact, is the competition it would have with the Baltimore City (convention center). I would have been more comfortable if there had been county money, not private money, for thihs center."

Throughout the day Prince George's County delegates were stopping legislators in the hallway trying to persuade them to vote for this bill, which they described as essential to the economic revitalization of the county.

The county has contended that with the convention center and accompanyin g hotels and a shopping center there would be at least 2,500 new jobs in the county and in five years at least $2 million increase in county revenues through taxes.

Much of the strong pressure was brought to bear because the District of Columbia is trying to push through its own convention center, which would be in direct competition with the Largo proposal. The District proposal woul cost more than three times as much as the Prince George's proposal, and it would be completely financed by federal funds.

For the past six weeks, O'Malley along with Levitt, architects and engineers of the George Hyman Construction Company worked intensely to put together maps, architectural drawings and detailed plans to present to the committee.

Apparently, though, only one delegate was sufficiently impressed with the presentation. Del. David Scull (D-Montgomery) who earlier had been skeptical of the scheme, said that after the presentation he was convinced that the convention center would provide needed economic stimulus to a country that had not received the benefit of federal installations like Montgomery County.

But it was this "take-it-or-leave-it" package that most disturbed members of the committee. Some wanted the state to have greater control over the kinds of contracts that would be let to construct the convention center.

John Lally, a spokesman for County Executive Kelly, appeared stunned by news of the defeat and at first had no response.

Later he suggested that "perhaps the committee required a more indepth presentation than we were able to provide due to the press of time. We will consult with the leadership of our delegation to determine what supplement data would be required and seek their advice on chances for a reversal.

"The concept for the convention center in Prince George's County has been and still is a sound idea and under our proposal it would help shift the (tax) burden off thje single family homeowner."

"The state bonds are essential for this project," Lally said, "and we are going to make every effort to maintain this proposal."

Backers of the center still have a chance, although the committee vote will undoubtedly weaken it.

The Appropriations Committee could reconsider its vote, but the lopsided defeat today makes that a slim hope.

Mandell has so far not committed himself publicly to the Prince George's County convention center.