All four Democratic candidates for mayor have tentatively agreed to take part in at least one live televised debate in the final days before the Sept. 14 primary election, which some observers believe could be crucial to the outcome of the race.

The tentative agreement capped a day of political jockeying mainly involving principal challenger Patricia Roberts Harris, whose aides consider a televised showdown critical to her campaign, and Mayor Marion Barry, who was the only candidate not to agree yesterday to either of two proposed formats.

Barry had rejected an earlier proposal to meet Harris in a one-on-one debate, claiming it would be unfair to the other two Democratic candidates.

Yesterday, under increased pressure from Harris, Barry's top staff members agreed in principle to a debate but deferred a final decision until they could decide which of the two proposed formats would be most advantageous.

Harris, rebuffed in her earlier call for a two-way debate, proposed a four-way debate at a press conference yesterday morning at her campaign headquarters downtown.

Several hours later, WJLA-TV Channel 7 offered to broadcast the Democratic debate on Sept. 8 from 9 p.m. to 10 p.m.

Harris and City Council members John Ray and Charlene Drew Jarvis, the other candidates, said through aides they would take part in the Channel 7 TV broadcast.

Barry's campaign manager, Ivanhoe Donaldson, said the mayor was willing to appear on any televised forum involving all four Democratic candidates, although he wanted to see the details of the Channel 7 program before making a final decision.

"Just as we've been doing all along, we wouldn't miss an opportunity if it's reasonable," Donaldson said.

"We've been debating out in the community at 75 to 90 forums. John Ray and Mrs. Jarvis have made most of them . . . If a TV debate comes along, we've yet to turn one down."

Later, Donaldson said that Barry might be more interested in taking part in an extended late evening program proposed by WDVM-TV (Channel 9) tentatively scheduled for Sept. 10, which would include all four candidates but have a more informal format.

Harris, whose aides think she is a better debater than Barry and would excel in a more structured setting, quickly accepted the Channel 7 proposal.

Thursa Thomas, director of public affairs at Channel 7, said yesterday that the ground rules for the debate must be worked out by officials of the station and the candidates.

She cautioned that "everything at this point is premature" because all the candidates have not formally accepted the invitations.

In a letter to the other candidates, released at her press conference, Harris said she was "convinced that the public still has not been given a fair opportunity to hear the candidates really address the critical issues facing the District of Columbia.

"I believe it would be a tragedy," she said, "if the public were denied this opportunity for a televised debate . The campaign should not end with three more weeks of mere rhetoric."

In the close three-way Democratic primary in 1978, there were several televised debates in the final days of the campaign.

Many political observers believe that the relatively poor performance of front-runner Sterling Tucker in those debates, especially one broadcast two days before the election, was a major factor in his narrow loss to Barry.