The D.C. Board of Elections and Ethics decided again yesterday that there cannot be a referendum Nov. 6 on a proposal to block financing of the downtown convention center. A Superior Court judge ruled hours later that the board made the right decision.

After being dealt the double setback, lawyers for the Convention Center Referendum Committee said they would make a last-pitch effort to force a vote by asking the D.C. Court of Appeals to reverse yesterday's actions.

Committee lawyers said a quick decision is needed because under the city's new referendum law, the deadline for the elections board to certify measures for the Nov. 6 ballot is this Thursday, 90 days before the election.

The dispute over building the $99 million convention center at Mount Vernon Square has become a high-velocity legal and administrative Ping-Pong match. Within the last week alone, the issue has been before the elections board twice and before Superior Court Judge Fred B. Ugast three times.

Ugast ruled last Tuesday that, contrary to a law enacted by the City Council in March that sought to bar all referendums on city-financed construction projects, they may be permitted under some circumstances.

This may be done, Ugast said, on projects for which Congress has not yet appropriated funds. Once the money has been provided, he said, it is too late.

Congress has appropriated $27 million requested by the city in 1977 for the convention center. It is in the process of providing another $45 million to permit construction to start next year. A referendum in November could affect only the last $27 million that would eventually be needed to complete the project.

When the referendum committee collected more than 15,000 signatures on a petition last fall, it wanted a public vote to block the whole convention center project, fearing the center would be a money-loser and a burden on taxpayers.

After being turned down twice by the elections board on its requests for a referedum, the committee took the matter to court. Following Ugast's ruling of last Tuesday, the committee rewrote its referendum proposal, asking that the elections board approve a public vote on only the last $27 million in center funds not yet appropriated.

At a special session convened yesterday on Ugast's instructions, the board said "no."

It found that the revised proposal for a referendum is so radically different from the one for which petitions were circulated that it could no longer qualify for the ballot.

Ugast agreed. Delivering an opinion from the bench, he said a question put to public referendum must be substantially identical to what was proposed when signatures were gathered on the petitions. What the committee is now proposing, he said, is "a distortion of what the court believes to be a fair and reasonable reading of the (original proposal)."