Sen. Patrick J. Leahy (D-Vt.), renewing his criticism of the planned downtown convention cemer, told city officials yesterday to give the Washington public another chance to be heard before the project moves forward.

Officials "should consult with civic associations and other citizens groups," Leahy said at a hearing on the city's budget for fiscal 1979. He observed that his mail indicates the project "is not popular with the citizens of the District of Columbia."

"There is no question, if put to a referendum in the city, it would probably lose," Leahy said.

As chairman of the Senate District Appropriations Subcommittee, Leahy said he agreed conditionally last week that Congress should release money for the center "to show our commitment to home rule."

A Senate-House conference committee, with Leahy as chairman, agreed to provide the first $27 million for the project if private investors will agree in advance to build millions of dollars worth of tax-generating hotels, restaurants and other facilities to serve convention visitors.

Under the unusual agreement, the Senate and House District Appropriations Subcommittees will release the money only after the city government certifies that the investors have firmly committed themselves.

The House of Representatives ratified the agreement by a narrow margin this week. The Senate will consider it today or next week.

When the city presents its revised proposed to the subcommittee, Leahy said there will be "a very thorough investigation."

Leahy's call for the city to let citizen groups have a voice in the new plan goes beyond the formal agreement that was reached by the conference committee.

Ben Gilbert, D.C. municipal planning director, told a reporter after the hearing that he agreed "there is a point in the process when citizens will have a chance to heard . . . we will stitch something into the process."

During the hearing, Leahy warned two officials of the Washington Area Convention and Visitors Association that they may have violated federal law in their lobbying for the convention center.

The center got a $200.000 appropriation from the District last year and a $100,000 outlay is proposed in next year's budget to promote tourism. That money is mingled in the association's treasury with funds collected from Washington businessmen.

Leahy said it is illegal to spend appropriated money for lobbying, and "you run the very serious risk of any taxpayer raising the issue."

"We'll have to change our book-keeping," replied Andrew Ockershausen, president of the association and general manager of WMAL radio. Austin Kenny, the association's executive vice president, told a reporter that the lobbying was limited to communicating with Congress and that Leahy "was digging pretty far down" to make the charge.