Two weeks before he won the Democratic presidential nomination, Jimmy Carter told the nation's mayors that cities were America's No. 1 economic problem.

"I pledge to you," he told the U.S. Conference of Mayors, "that if I become president, you, the mayors of America, will have a friend, an ally and a partner in the White House."

On Monday, some 21 months later, the mayors will learn just what kind of friend they have, when Carter announces the results of his administration's year-long effort to devise a program reversing the downward spiral afflicting many cities.

If the expectations voiced this week in some quarters here prove correct, the plan will be seen as something less than cosmic.

One representative of city interests predicted, "The money will probably be peanuts, and we'll get the shells."

Rep. Henry S. Reuss (D-Wis.), chairman of the House Banking, Finance and Urban Affairs Committee, said he was "not expecting people to cry 'Excelsior' when this plan is announced, but I intend to be thankful for small favors. I consider it a first working draft, and I think Congress will have to produce phase two fo the plan."

The pessimism may be unwarranted, but it has been fostered by a variety of signals from the administration.

First, there's the question of money. White House aides have said that budget constraints will limit the amount going for new urban initiatives.

Reuss said he expects Carter's message to call for about $2 billion extra, and Rep. Robert N. Giaimo (D-Conn.), chairman of the House Budget Committee, told the National League of Cities recently that Congress probably would not approve more than that.

The White House has also stressed that the federal government already gives about $85 billion to states and localities, and the Advisory Commission on Intergovernmental Relations has noted that federal grants to 15 of the nation's largest cities average 47.5 cents for every dollar raised locally.

But one key urban strategist says those statistics distort the picture. He notes that $28 billion of the $85 billion is in welfare payments to individuals, and that of the $57 billion remaining, $32.5 billion goes to local governments - towns, cities, and counties. "The nation's 47 largest cities get about 20 percent - $6.5 billion - of the $32.5 billion," he said.

The point, though, is that whatever Carter requests in next week's message, it will add only a small increment to what urban areas are already getting.

Second, there's a question of focus. Last December Carter's urban planners were stressing the so-called "worst-case" cities, and they drafted a policy memo that included the objective of "helping reduce fiscal and social disparities between cities and their suburban neighbors."

But they quickly found that such a program could not pass Congress. While 73 percent of the American people live in urban areas, only about 30 percent live in central cities - and they don't have the clout to get approval for programs that seem antisuburban.

So the controversial language on disparities was removed from a 10-page policy statement Carter signed in January.

To charges that the change represented a copout, Housing and Urban Development Secretary Patricia Roberts Harris says, "It'll be a copout only if we don't target the money to places that need it most."

Third, there's a question of priorities. Some urban advocates don't think an urban plan ranks very high in an administration that has been grappling with a coal strike, the Panama Canal treaties and the Middle East.

The Carter plan is expected to call for:

A redesign of counter cyclical aid to communities with high jobless rates. Under the antirecession program, which is due to expire in October, $1.5 billion in federal money is going to states and localities this year. The questions are how much money to include in a new program, whether it should go only to localities and, if so, which ones.

Incentives to states that ease the financial burden of their cities. Some aides have suggested that Carter request $200 million for the program; others have suggested that Carter request $200 million for the program; others have suggested $400 million.

A program of "soft public works", or maintenance and repair projects that would create public service jobs. Such work could include repair of streets or the roof on city hall.

Use of federal incentives to encourage business investment in cities. Carter has been given options of setting up a national development bank in the Commerce Department or in HUD or as an independent agency to make loans or loan guarantees for such investment. Some urban experts have argued that the bank is not needed, and Carter could reject the proposal but recommend increased fund as for development incentive programs that already exist in HUD and Commerce.

Extension of procurement policies that favor minority businesses to other federal grant programs besides the one for local public works. That policy says that at least 10 percent of supplies purchased for the projects have to come from minority-owned firms.

Better coordination of programs that affect urban areas.

Perhaps the most important facet of Carter's new program will turn out to be the administration's extensive review of the current 448 federal grant programs to states and localities.

The review, according to one urban planner, has shown officials how current program can be turned around so that they promote urban growth rather than subsidize suburban sprawl.

"If this exercise accomplishes nothing else, said Reuss, "it may force government to cease doing that which hurts cities, and that alone will confer a great boon on the people that live in them."