A D.C. Superior Court judge opened a way yesterday for foes of the downtown convention center to get a referendum that could upset the financing of the project after it already is under construction. But he refused to order a citywide vote that could block its start.

Walking a judicial tightrope down the middle of a complex political and economic dispute, Judge Fred B. Ugast ruled that referendums can be conducted on projects for which Congress has not yet appropriated funds.

But after the money has been appropriated, Ugast declared, it is too late for such a public vote.

Opponents of the convention center, who brought yesterday's court action in an effort to win a referendum on the total project, immediately said they would seek a referendum challenging the center, based on Ugast's ruling.

A successful referendum initiative at this stage could block the city from obtaining the final $27 million it will need to complete the $99 million complex. Already $72 million for the project either has been appropriated by Congress and spent or is in the current District budget which is expected to be approved by Congress shortly.

The city already has spent $27 million, mainly to buy three blocks of land near Mount Vernon Square as the center site.

James C. McKay Jr., an assistant corporation counsel who argued the case before Ugast, would not discuss his interpretation of the ruling. Later, James O. Gibson, the city's planning chief, said after talking to McKay he believed yesterday's ruling would prevent any referendum on the money needed to complete the center.

John J. Phelan, a leader of opposition to the convention center, said he will ask the D.C. Board of Elections and Ethics today to certify a referendum Nov. 6 seeking to block the city from asking for the last $27 million.

Phelan is chairman of the Convention Center Referendum Committee, a citizen group that brought the lawsuit that resulted in yesterday's ruling by Ugast.

Phelan said he will ask the elections board to certify petition forms already circulated and signed by about 15,000 voters.

Ugast, in yesterday's verbal ruling from the bench, said the petition forms were worded properly for seeking a referendum. However, Ugast said, a more detailed referendum measure that was submitted along with the petitions was legally flawed because it sought to block the spending of appropriations already made.

William Schultz, lawyer for Phelan's group, said the measure would be redrafted so that it can challenge only future spending.

Phelan and his allies, including many members of neighborhood civic organizations, have contended the center would be a money-loser needing heavy subsidies.

Ugast, in yesterday's ruling, struck down an amendement written into the city's referendum law last March by City Council Chairman Arrington L. Dixon and approved by a council vote of 6 to 5. Dixon's amendment would have flatly prohibited a referendum on any proposal for a city-financed construction project.

The elections board leaned heavily on Dixon's amendment in refusing earlier to certify a referendum on the center, leading to the lawsuit.