Last Tuesday, W. Reid Thompson, president of the Potomac Electric Power Co., contributed $1,000 to Mayor Marion Barry's reelection campaign. Within 48 hours, nine other company executives also made private contributions, topped by a $2,000 donation from the company itself.

The sudden $6,750 shower of cash was bestowed on a candidate who publicly supports a proposed gross receipts tax that could cost Pepco millions of dollars each year, who is perceived as so weak that a host of challengers has lined up to try to unseat him and who has been shown by at least one poll to be starting from an unusually low base of support for an incumbent.

Pepco officials have said that not all the contributions were coordinated and that the company did not base its $2,000 corporate gift on a single issue, but rather approved Barry's overall performance, believed he has done well and wanted to back him and "to stay in touch with him."

In a sense, the Pepco contributions may be viewed as a tribute to the cash value of incumbency and its ability to generate funds--less on the basis of personal affinity, or ideological similarity, than because of a pragmatic recognition of the need to keep open the channels of communication, at least to stay on speaking terms with power. To "stay in touch."

The Pepco contributions are only one example of the way Barry, four years ago an outsider, has been able to capitalize on his current status and amass in seven weeks of fund- raising more than $378,000, a figure that might be expected to intimidate the most stout-hearted of his present and prospective campaign foes.

With the critical Sept. 14 Democratic primary still six months away and the campaigning in earnest yet to begin, following the footprints of contributors is one way to monitor the race.

Since he filed as a candidate in late January, Barry has set a breathtaking, breakneck fund-raising pace. Using personal calls and the persuasive powers of a finance committee of 100--each pledged to give or raise $2,000--Barry the incumbent has raised in less than two months virtually as much as Barry the underdog attracted during the entire 1978 primary and general election campaigns.

In the 1978 primary, Barry's list of campaign contributors was largely a patchwork of individuals and small firms that he scrounged up from outside the city's political mainstream. This year Barry's contributions are coming from the likes of Pepco and from such well-known people as former Supreme Court Justice Abe Fortas and his wife, who gave $750.

City Council member John Ray, also a candidate for mayor, once clerked for Fortas' law firm, counts him as a close personal friend and has said he considered Fortas almost a surrogate father.

"Polly Shackeleton council member from Ward 3 and the Fortases are good friends and Polly is a supporter of the mayor," Ray said about the Fortas contribution to Barry. "The mayor also gave Fortas a low license tag, so I guess he felt he'd make the contribution."

The mayor recieved large amounts of money from three general categories of contributors:

* Persons or firms seeking to do business with the city gave generously. Herbert Haft, who owns Dart Drug Company and is competing for the rights to develop city-owned property at 14th Street and Park Road, contributed to Barry $1,000 through Dart Drug; $2,000 through Combined Properties Corp.; and another $1,000 through 3301 Pennsy Drive Associates. Haft also held a fund-raiser for the mayor at his home in Bethesda last Tuesday, netting about $40,000, according to Ivanhoe Donaldson, Barry's campaign manager in 1978 and currently the city's acting director of employment services.

* Companies with current contracts with the city also gave to the mayor. Metro Chemical, which has $20,000 worth of business with the city, gave $600. American Motohol Supply Company, which supplies heating oil to the city, gave $500.

* Government workers were the most frequent contributors of individual donations to the mayor, usually in amounts of $50 or $20. That was the cost of admission and guest admission, respectively, to the mayor's birthday party at Georgetown Park, the new shopping center on M Street NW, on March 6. Some government workers gave even more. Budget Director Gladys W. Mack, for example, gave $500, and former human resources director and now city computer aide Joseph P. Yeldell chipped in $80.

The rapid rate of giving to Barry's campaign is being attributed to his incumbency. Opponents say it is fear of vindictiveness that is pumping up the mayor's campiagn treasury.

They say the mayor has made it clear to the businessmen and lawyers who are the main source of campaign contributions that if they do not support him they will get no red tape broken for them as their requests go through the city government, no special treatment when they ask for favors or ask to have their phone calls returned.

Barry says he has not pressured anyone to contribute and says he is pleasantly surprised that government workers, for example, have been calling him to give "generous contributions."

"People like me," the mayor said in a telephone interview. "That's why they give. We don't have to twist any arms. They believe in me and think I'm the best candidate for the city . . . . they also know that I don't mix city business and politics in any way. If anyone gets any contract or anything else from the city it's because it's good for the city, they're competent and it meets city law."

The mayor said the Pepco contribution and the money raised for him by Herbert Haft came about because he is "the leading candidate," and not because the donors would like to curry favor with the District government.

"I'm the one to beat," he said in a telephone interview from his home Saturday. "I've got to be the favorite in anyone's view. I know I've had some difficulties, I've made mistakes from time to time, but I've learned, and they know that. If you try to trace everyone's motive you are going to get all twisted."

With the $378,275 he has raised, the mayor is far ahead of his nearest competition for campaign dollars, according to reports filed with the city's Office of Campaign Finance last week. Ray is the closest with $226,937. But in the last six weeks Ray raised only $22,795, a sharp decline in the rate of contributions to his campaign.

Even worse for Ray, he has now spent $171,811, more than two-thirds of the money he has taken in, on radio and bus advertisements as well as on fees to political consultants. His February payroll alone was $12,844.91. At that rate, he would have to collect $83,491 just to pay his current staff through the primary.

"In my view," said Ray, "we're going to raise $500,000 and we're going to spend that money about as fast as it comes in, because we have to make a quick, grass-roots show of support....The question is, when we reach May will the money spent back here mean anything both in name identification and in organizing my campaign?"

As Ray's rate of contributions has declined, council member John A. Wilson's rate of donations also went down. Wilson had raised about $75,000 thourgh January--not including about $30,000 raised by a committee to explore his chances as a candidate--but in the last six weeks of contributions did not get past $11,469.

Wilson, like Ray, says that he is not worried by the decline in the rate of giving, attributing it to a slow cycle in a campaign that has yet to get underway. Wilson did attract some new contributors, notably Bishop Smallwood E. Williams of Bible Way Church, a strong figure who influences a large congregation; and two bankers, Sam Fogie Sr., president of United National Bank, and B. Doyle Mitchell, president of Industrial Bank. All three gave $100 contributions. Wilson, again like Ray, has spent most of the money he has collected. Wilson has spent over $55,000 of his total $75,000, about $31,000 of it for television commercials.

Council member Betty Ann Kane, meanwhile, staged a comeback from the first reporting period, when she finished last in attracting donations with a relatively small $24,860. In the last six weeks she more than doubled that amount, adding $55,522 to her campaign treasury.

Kane, spending at a much slower pace than either Ray or Wilson, has $61,625 still on hand. Wilson had about $20,000 in ready cash remaining by last Wednesday while Ray had $54,000 in ready cash.

"One of the reasons I'm very pleased at the strong pick-up in fund-raising," Kane said Saturday, three days after her first radio commercials began for about two weeks at a price of $7,500, "is that it helps to convince people that I am a viable candidate. The campaign now has some momentum and is going somewhere while other candidates are losing momentum."

About half of Kane's donors in the last six weeks have been lawyers. Ray, a lawyer, was expected to do well among this group, but Kane's performance here was unexpected. Kane's husband, Noel, is a lawyer and two members of her finance committee, Arthur J. Rothkopf and John R. Risher Jr., are lawyers.

"In Betty Ann's political career she has never needed to raise a large amount of money," said Kane's campaign manager, Dianna Brochendorff. "We don't have any established brokers laying out our plan for getting money. When I first came here I wasn't too comfortable with how the money was going, but now I'm comfortable."

While Kane took over two months to get $80,000, former Carter cabinet secretary Patricia Roberts Harris raised that amount in three weeks. Harris, who is scheduled to open her campaign Saturday, had spent only $5,405 of that money by last Wednesday, leaving her with the most cash on hand of any of the challengers.

Last week Harris' camapign director, lawyer Sharon P. Dixon, confirmed that G. William Miller, secretary of the treasury in the Carter administration, and Smith W. Bagley, a major Washington real estate broker, will be raising money for Harris.

Very little of Harris' money came from outside Washington. The bulk came from Washington lawyers, many of whom are associated with Harris through her former law firm, Fried, Frank, Harris, Shriver & Kampelman. That firm, along with the firm of Wilmer & Pickering, accounted for over $20,000 in donations to Harris.

In addition, some well-known lawyers contributed to Harris, including William T. Coleman, a former secretary of Transportation in the Ford administration, and Nicholas Katzenbach, who was U.S. attorney general under President Johnson and now is vice president and general counsel for IBM corporation. Harris serves on that company's board of directors.

Harris also received a $2,000 contribution, the maximum allowed by law, from James W. Rouse, a developer who built the city of Columbia, Md., and Harbor Place in Baltimore.

"We didn't really start fund-raising until the 23rd of February ," said Dixon, "so this wasn't a very organized effort. We haven't had a formal finance committee in place yet but we're setting that up now and that will make fundraising easier . . . but what we have is enough money to justify a campaign and carry out a good campaign." CAPTION: Picture, Mayor Marion S. Barry: His $378,000 campaign cup runneth over. By Ray Lustig -- The Washington Post