In his strong public statement on the issue to date, Sen. Patrick J. Leahy (D-Vt.) has warned District of Columbia officials that Congress will not veto a formal city budget for this year unless the city drops or drastically changes its proposal for a downtown convention center.

Leahy, chairman of the Senate D.C. Appropriations Subcommittee, issued his warning in a letter made public yesterday, at almost the same time a report was released by D.C. budget direcor Comer S. Coppie detailing the effects of not having a budget.

Although some individual departments and programs might be hobbled - notably a planned step-up by the Department of Human Resources of the monitoring of welfare rolls - Coppie's report indicated only a moderate effect on routine municipal operations.

However, the city's construction programs have been deeply hit, Coppie reported. Of $177 million in proposed projects, the city is able to proceed on only $15.9 million, chiefly involving street and building repairs.

City operations are being financed under a stopgap resolution enacted by Congress in December that permits the city to spend at about last year's levels until the end of the 1978 fiscal year next Sept. 30.

Leahy, in a letter to City Council members, said bluntly that "the convention center question has totally deadlocked any additional progress on the 1978 budget."

The city has asked Congress to provide $27 million, chiefly to buy land, for a project estimated to cost $110 million in the Mount Vernon Square area.

The House voted the funds, but the Senate - at Leahy's urging - refused to go along. That led to a deadlock of House-Senate deliberation on the final shape of the budget.

"It now appears," Leahy said in his letter, "that unless the city either withdraws its current convention center request or prepares an amended proposal to address the objections to the current proposal, the budget will remain deadlocked for the remainder of the year."

Two City Council members, Nadine Winter (D-Ward 6) and Douglas E. Moore (D-At Large), announced plans separately yesterday to withdraw the convention center fund request. Winter held a news conference to appeal to citizens to write or telephone city hall in support of her position.

There were no signs, however, that Mayor Walter E. Washington's administration was wavering in its firm support of the project. Coppie said he knows of no plan to withdraw the budget request.

For city officials, the issue involves some sensitive congressional relationships.

Rep. William H. Natcher (D-Ky.), the veteran chairman of the House D.C. Appropriation Subcommittee, put his prestige on the line in supporting the center, and could be expected to resent any reversal by the city.

Leahy, on the other hand, is acruing his first year as the subcommittee chairman. After winning Senate support for his opposition to the project by a vote of 65 to 25, he has made it clear that he will not go back to the Senate's elders to seek a reversal.

Coppie told a recent news conference that it would be unthinkable for Congress not to enact a formal budget.

Yesterday's report by Coppie drew no such conclusions. It was a bland compilation of figures and legal citations. At it applied to routine city operations, its main point was that the budget requests that the stopgap resolution is actually providing most of what the city needs.

The proposed operating budget is $1.2 billion, an increase of $181 million over last year. The stopgap resolution permits spending an additional $128 million of that increase, the report said.

One reason is that the city is permitted to spend above last year's levels to meet the cost of higher wages for city employes.

The report concluded that the cost of construction projects delayed by the deadlocked will be increased by 10 percent, or $20 million, if the money is not released.