The D.C. branch of Common Cause charged yesterday that all three major Democratic mayoral candidates in the Sept. 12 primary are violating the city's election law by not reporting the occupations of all their contributors who give $50 or more.

The lobbying group said an examination of campaign finance reports filed through June 10 showed that 26 percent of contributions of $100 or more to Mayor Walter E. Washington's reelection campaign - a total of $6,650 - did not list the occupation of the donor.

Common Cause said that occupations for 20 percent of those who contributed $100 or more to City Council Chairman Sterling Tucker's mayoral campaign - a total of $20,160 - were not listed, while occupations for 16 percent of contributors of $100 or more to Council member Marion Barry's campaign - totaling $11,250 - were not reported. The group also said that 45 percent of those contributors who gave $100 or more to John L. Ray's mayoral campaign - $2,200 - were not listed.

By contrast, Common Cause said that the two chief rivals for the Democratic nomination for City Council chairman, Councilmen Arrington Dixon and Douglas E. Moore, have listed all the occupations of their contributors who gave $100 or more.

Jeff Bartholet, a Common Cause staff member who compiled the report, said the violations should not be viewed merely as technical ones.

"What it means to us is that you can't tell what special interests are supporting the candidates," he said.

The D.C. Board of Elections and Ethics could fine the respective campaigns up to $50 a day for each day their campaign finance reports are not complete. But the Washington, Tucker and Barry campaigns all said that no one from the Board of Elections has called them to ask for more information on the occupations of their contributors.

No election official could be reached to determined what, if anything, the board plans to do about the apparent violations.

Lacy Streeter, Washington's campaign manager, said the elections board told his campaign to simply note at the bottom of the contributions report when checks were received by mail and the donor's occupations were not known. Streeter said the occupational information is then sought and submitted to the elections board as it is received.

Gerald Wallette, Tucker's campaign manager, said the occupations of some donors are difficult to list because the contributors are retired, do not work or are on vacation. He said donors are called to try to learn their occupations.

Florence Tate, Barry's spokeswoman, said that where "it doesn't take detective work" the Barry campaign also seeks the occupations of the donors. She said people who do not list their occupation on contribution cards "are not people who are trying to get close to the candidate. People who are connected with a special interest always want you to know."