A nationally prominent political consulting firm has withdrawn from Mayor Marion Barry's reelection campaign after a disagreement over when Barry, who has scored unusually low for an incumbent in early polls, should begin actively campaigning.

Marttila and Kiley, a Boston firm which has advised Detroit's mayor Coleman Young, Boston's Kevin White and New Orleans' Ernest N. (Dutch) Morial, had already done a poll for Barry's campaign.

The firm notified Barry about two weeks ago, however, that it no longer wanted to be involved in the reelection effort. "It's clear our conceptual approach to the campaign was not shared," John Marttila, a partner in the firm, said yesterday.

The disagreement centered largely on when Barry would begin actively campaigning openly instead of campaigning indirectly through high-profile official duties as the incumbent mayor, according to knowledgeable sources.

Ivanhoe Donaldson, Barry's chief political adviser, said yesterday that Marttila had proposed the campaign begin activities that it was "in no position" to start, but said there was no basic disagreement on strategy. "There was no rejection of his ideas," Donaldson said.

Marttila had proposed that the campaign begin efforts immediately to overcome a high level of negative voter reaction to Barry found in a poll taken by the organization in February, according to sources.

Marttila and Kiley is known for emphasizing personality, often through television advertising portraying friendly, family-loving candidates, to counteract negative images.

Sources said the firm felt Barry needed to begin work on his image immediately, as well as to address issues that would distinguish him from Patrica Roberts Harris, a former Carter administration cabinet member, who early polls indicated was Barry's strongest challenger in the Sept. 14 Democratic primary.

Barry's aides have said the mayor wants to delay becoming identified as a candidate in the minds of voters, in order to retain his standing as mayor--with all the benefits of incumbency such as ready access to the press--for as long as possible, while his opponents, now numbering eight Democrats, scramble for attention.

"The mayor is clearly out there working hard, so I don't think that was the problem," said Donaldson, who is currently acting director of the D.C. Department of Employment Services but is scheduled to quit that post next week and run the Barry campaign, as he did in 1978. "We're organizing now. He's not sitting behind his desk trying to hide from anyone."

"John Marttila expressed some frustrations about the campaign to me over the phone," Donaldson said. "What he wanted was for me to be available to him. I said I was still doing this job and it would come all in good time."

In a telephone interview, Marttila said that technically his firm was not withdrawing from the campaign, because it had never signed a formal agreement for consulting services with the Barry reelection committee. He acknowledged, however, that his firm had decided unilaterally to end its relationship with the campaign.

"A good consulting relationship has to have the right chemistry; harmony is necessary," Marttila said. "Sometimes it just doesn't work and a person has to be mature enough to recognize that."

The firm, with more than 12 years of experience in running campaigns, has never withdrawn its offer to work for a major campaign before, he said.

Donaldson said he thinks Marttila and Kiley still may work with Barry's campaign once he takes over the campaign and speaks to them directly. He said he had not contacted them since receiving the letter.