The U.S. Conference of Mayors yesterday attacked President Carter for abandoning earlier commitments in his new budget and promised to resist the president's drive to shrink the federal government's share of the economy.

John Gunther, executive director of the conference, which represents 800 large cities, said Carter appeared to be backing away from his own urban program and said further efforts to restrain government spending would wipe out new initiatives for national health insurance and welfare revision.

Gunther, speaking on behalf of the conference, also said he doubted that Carter's "anti-inflation budget" would reduce inflation, charging that the president's program does little to attack such root causes of inflation as food, fuel, housing and health -- which make up 70 percent of a family's household budget.

Finally, the mayors, in an 87-page analysis, denounced Carter's "apparent abandonment of federal programs designed to stimulate employment in the private sector." They cited Carter's failure to seek funds in 1980 for a private-sector job program, funded at $400 million this year.

The Carter budget "is based on economic and social principles which are a disappointment to the cities," the report said.

The mayors begin their annual midwinter meeting here today.

Traditionally, the big-city mayors have called for more federal spending, but for the first time in recent memory they did not ask for new programs or for an overall increase in the budget. Last year they sought $11.3 billion extra for new urban and job programs.

This year, Gunther indicated, they will align themselves with civil rights, labor, business and other urban groups to lobby for specific domestic spending increases. He indicated he felt proposed increases for military spending ought to be cut, declined to say where.

The philosophical split was considered significant because the conference in recent years has been dominated by Democrats. Now 40 percent of its mayors are Democrats, 30 percent are Republicans and the rest are independents.

Gunther alluded to a speech earlier this month by Carter's chief domestic adviser, Stuart E. Eizenstat, who said that "new realities" require constraints in social programs.

"I don't believe anyone in the administration or out can tell you how the president's policies are supposed to bring about economic prosperity in this country, and maybe that's what the new realities are all about," Gunther said.

"Maybe the new realities are that our country can't have prosperity, that we're always going to be just not quite whole. I don't believe that's the case. I think the country is greater than that."

Adked if he thought Carter's urban program -- most of which was not passed by Congress last year and some of which is not being proposed again this year -- is "a fraud," Gunther replied:

"I think we're in a holding pattern. We're in a pause."

Gunther criticized Carter's effort to reduce federal Spending to 21.2 percent of the nation's economic output in fiscal 1980 and his ultimate goal of getting it to 20 percent. With fixed costs like Social Security going up, the administration would have to "do away" with such programs as community development, economic development and manpower programs to make room for health insurance or welfare revision, Gunther said.

Such constraints would reduce the likelihood that the administration would take on such initiatives, he added.

Besides criticizing Carter's failure to seek new funding for the private sector job stimulus program, Gunther charged that the president was seeking insufficient funds for urban parks and that the Economic Development Administration was being cut back by 4 percent in its regular grant and loan programs.

He also chided the president for not proposing a program for public works maintenance jobs such as repairing city sewer systems or public buildings. Last year Carter proposed a $1 billion, three-year program that would emphasize hiring long-term unemployable people, but Congress did not pass it.

Gunther pledged that the conference would work with Carter to pass his proposal for antirecession aid to distressed cities and for a national development bank to spur business and create jobs in rundown areas. Those proposals also failed last year.

But Gunther warned that unless Carter works as hard for the measures as he did for the Panama Canal treaties, "they won't happen"