One can only wonder why the Republicans made such a fuss over Mayor Marion Barry's decision last week to release a U.S. Conference of Mayors report on the plight of the cities, only a few days before the presidential election.

Three Republican mayors, including the president of the conference, issued statements criticizing Barry, a Democrat, for trying to embarrass President Reagan with a last-minute release of the study attacking the administration's urban policies and proposing $49 billion in new federal programs.

"I can say without hesitation at all that this particular activity constituted the most blatant use of the Conference of Mayors . . . for purely partisan political purposes in memory ," said Mayor Robert Isaac of Colorado Spring, one of the three Republican critics. Actually, those mayors should have been rejoicing.

In the face of a rising conservative tide against big government spending and tax increases, Barry's report calling for massive spending equal to a quarter of the federal deficit was merely more grist for the Reagan political mill.

Far from helping Walter F. Mondale's campaign, the report outlined in glaring terms the type of costly urban programs that many Democratic mayors have begun to shy away from.

There were other unusual aspects to Barry's handling of the report. The document was supposed to have been completed nearly a year ago, or at least in time for it to be reviewed by the full membership in Philadelphia. Instead, Barry waited until a week before the election to unveil it, and he did it on his own, with no other conference members present.

Moreover, the report, which was released as an official document of the U.S. Conference of Mayors, was published in the print shop of the D.C. Department of Corrections, at city expense. The mayor's office said that no figures are available on how much it cost to print the report.

Barry released the report as chairman of the Special Committee on a National Urban Policy. As such, he had led the lobbying at the annual meeting of the Conference of Mayors in Philadelphia in June in support of resolutions favoring major federal initiatives to help depressed urban areas.

A major item on Barry's agenda was a proposal for a National Urban Development Bank, designed to pump $20 billion in working capital into depressed urban areas over a four-year period. But a majority of the mayors didn't like the idea, and Barry's resolution was tabled. Yet when Barry unveiled his 420-page report last week, called "Plan to Rebuild America Cities," his urban bank proposal was the centerpiece.

The report contained a laundry list of other spending proposals, including $4 billion a year in federal assistance for targeted distressed areas; a new national employment and training policy that would lower the unemployement rate by at least 1 percentage point, and a major increase in funds for housing subsidies.

The study also rejected Reagan's proposal for establishing special urban enterprise zones.

Yet a resolution endorsing urban enterprise zones was adopted by the Conference of Mayors in June. Republican Mayor William J. Althaus of York, Pa., who sponsored the resolution, believes that Barry was attempting to make an end run in issuing a report that flies in the face of Conference of Mayors policy.

"I don't care what Barry says and he doesn't care what I say, but it the report is not conference policy," Althaus said at a news conference that he held here with Isaac last week.

Meanwhile, Mayor Hernan Padilla of San Juan, Puerto Rico, president of the conference, issued a statement warning, "I and many other mayors will not agree on a proposed $49 billion spending program for urban areas."