James Rouse, the wealthy Columbia developer who is co-chairman of Jimmy Carter's Maryland primary campaign, started his day yesterday with what by most measures was a minor event -- a quick breakfast for several dozen small Prince George's businessmen at Calverton's Ramada Inn.

Nevertheless, by the time Rouse reached the punchline of his speech -- an appeal for contributions to the Carter campaign -- a half-dozen of Maryland's elected officials were there to back him up. As Rouse stood by, both Lt. Gov. Samuel Bogley and Secretary of State Fred Wineland stood up to appeal to the small group, even as State Sen. Tommy Broadwater circled the room with tickets to a Carter fundraiser.

All their efforts met with little immediate success among the businessmen. But the scene was representive of Rouse's greatest resource in Carter's Maryland organization, which has been left without funds by national campaign officials.

Slowly but surley, the Carter campaign has begun to mobilize that over whelming part of the state's political establishment that has endorsed the president, and that is capable of bringing his campaign hundreds of thousands of dollars in contributions, and, Rouse hopes, enough votes for a convincing victory in the May 13 primary.

After yesterday's breakfast ended, for example, Rouse went to Annapolis to meet with Gov. Harry Hughes, who endorsed Carter in January but has done little for him since, Rouse and Ed Crawford, Carter's Maryland campaign coodinator, pressed Hughes to make campaign appearances in western Maryland and his native Eastern Shore -- where they feel his influence is likely to be greatest.

In addition to Hughes, Bogley, and Wineland, the Carter campaign has enlisted nearly 300 other elected state officials, a number of prominent businessmen led by Rouse, and several of the state's largest union.

The campaign chairman know that in 1976, a similar combination of these forces, orchestrated by former Gov. Marvin Mandel, handed California Gov. Jerry Brown a decisive primary victory over Carter.

No one expects an organizational victory of that kind to happen again. And supporters of Carter's challenger, Sen. Edward M. Kennedy, say that the many political endorsements will mean, nothing for an imcumbent president when the state voters make their choice.

But Carter's campaign is showing already that elected officials and large unions can be invaluable when it comes to at least one critical aspect of campaigning fundraising.

By March, the Carter campaign already has raised close to $200,000 in Maryland, largely through the work and connections of finance chairman and weathly Montgomery developer Nathan Landau. Now Rouse and Crawford are focusing on a massive fund-raising dinner planned for the Baltimore City Convention Center late this month -- and the state's politicians are going to work.

Baltimore Mayor William Donald Schnaefer has promised to sell a total of 350 tickets worth $12,500 to the event, and "has his people out selling tickets all over the city," according to aide Chris Hartman. Both state House Speaker Benjamin L. Cardin (D-Balt.) and Senate President James Clark (D-Howard) have taken $1,000 worth of tickets, and Balitmore County Executive Donald Hutchinson has more than $10,000 worth.

In fact, every elected official in the state who endorsed Carter is now carrying at least $650 worth of tickets to the president's fundraiser. Meanwhile, both the state Teamsters and Food and Commercial Workers unions have pledged to deliver more than $12,000 in ticket sales to the campaign.

"My life is a lot easier once we got all the elected officials on board," said Crawford.

Ironically, Carter's Maryland campaign organization probably will not be able to spend the money it raises, according to Crawford. The funds are channeled directly into the national effort, which, hard-pressed by a federal campaign spending ceiling, has decided that Maryland "is not a priority state."

"Our budget is zero at least until the last two weeks." Crawford said yesterday. "They've spent a lot more overall in places like Pennsylvania the spending limit is becoming a concern."

The lack of funds to buy media advertising means that Crawford and Rouse are now depending on state officials to give their campaign exposure and organization, as well as money.

Campaign trips by both Bogley and Hughes will draw free media coverage particularly in the state's rural areas. And state officials are being encouraged to hold organizational meetings and to make calls to their own loyalists.

Already, the advantages of such official organizing help are showing up in Carter's campaign. Shortly after he finished yesterday morning's breakfast, for example. Wineland agreed to stage a rally next weekend at his own home.

"I'll get 200 or 300 people there," he promised Rouse. "And you can come and speak to them."