Doubts the Reagan administration can meet its spending targets rippled through the House and Senate yesterday, prompting complaints about "mirrors-and-magic" budgeting and suggestions that Congress may make even deeper spending cuts than the president has proposed.

One idea that seemed to be gaining support would be to scale back the automatic cost-of-living increases for Social Security and other major federal programs, the "indexing" of benefits that is expected to cost the government $22 billion next year.

The day after President Reagan sent his billions in proposed budget cuts to Congress, nearly half his Cabinet was on Capitol Hill defending them before sometimes hostile congressional committees that were also hearing cries of pain from interest groups.

Labor Secretary Raymond J. Donovan was badgered so much by a House subcommitee about social welfare cuts that he blured out, "I disgree with you that this administration is heartless. . . . I personally resent it [the allegation]."

Education Secretary Terrel H. Bell was simultaneoulsy defending the administration's proposed budget cuts for public schools and its plan for tuition tax credits for students in private schools, and Attorney General William French Smith denied that Justice Department personnel cuts would impede law enforcement.

Amtrak officials said Reagan's cuts for rail passenger service would leave the system with 700 idle cars, many of them brand new. there may even have to be commercials on public television, said the president of the Corporation for Public Broadcasting.

The dominant theme in the two budget committees, where action will begin shortly on the Reagan proposals. was skepticism -- directed more toward which the new Reagan budget was built than toward the details of the budget.

"We are not going to put out a budget based on mirrors and magic," said House Budget Committee Chairman James R. Jones (D-Okla.) in questioning the economic forecasts that the administration used in projecting a $695.3 budget and $45 billion deficit for fiscal 1982.

"We are not going to put out a budget based on assumptions which six months or a year from now will be held up to ridicule," Jones added, noting at one point that Reagan was projecting interest rates at 8.9 percent for next year while most private economists are talking about 12 per cent.

Murray L. Weidenbaum, chairman of the Council of Economic Advisers, responded that the forcecast would turn out to be realistic if Congress recovery program, although he conceded that he was relying in part on what he called his "visceral computer model . . . the gut feeling of a forecaster with some 30 years experience. . . ."

At the same time, before the Senate Budget Committee, congressional budget director Alice M. Rivlin called the administration's economic assumptions "optimistic but not impossible" but added: "If the administration's economic scenario is not attained . . . the consequences for the budget are troubling. Higher inflation, higher interest rates and higher unemployment would all work to produce more federal spending and larger budget deficits."

The administration assumptions "leave no room for further price shocks for oil and food" or for a continued escalation of wage demands, she contended.

To avoid risks, she told a seemingly sympoathetic committee, it could make deeper cuts than Reagan proposed or "consider alternative spending cuts that would reduce the sensitivity of the federal budget to inflation," such as modifying the way that Social Security and other benefits are adjusted for inflation.

Senate Budget Committee Chairman Pete V. Domenici (R-N.M.) told reporters later there is "genuine interest" in Congress in indexing changes. Although Budget Director David A. Stockman has opposed such changes, Domenici said he thought the administration might go along if it gets the total amount of cuts that it is seeking. Jones, the House budget chairman, has also expressed interest in modifying the cost-of-living adjustments.

By cutting back on these adjustments, Congress could avoid some of the painful program cuts that Reagan proposed or achieve greater savings as a hedge against faulty economic forecasts and bigger deficits. But tampering with Social Security is politically dangerous, and some lawmakers say it would be a futile exercise without administration support.

Meanwhile, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) continued to warn that the prevailing support in Congress for heavy budget cuts may well change when the country realizes their full impact. "I think the average person on the street has no concept of the severe changes in policy of the Reagan administration," he told reporters.

O'Neill said the Democrats are trying to work out a set of principles that "we hope all members in our party can agree on," although aides said the comment did not signal a departure from the Democratic leadership's pledge to work with the administration in fashioning a cooperative party together" on the basis of basic Democratic programs such as health, education and food services, O'Neill added.

Even Republicans are not unanimous in supporting the program cuts, the speaker said, noting that Republican as well as Democratic opposition is expected when the administration's plan to limit diary subsidies hits the House floor later this month.

On both sides of Congress, there were continued expressions of concern that Congress was being asked to act without adequate information on details of program cuts as well as the economic assumptions upon which they are based.

At the House Budget Committee, both Republicans and Democrats warned that the White House risks losing major chunks of its proposed restructuring of federal aid to schools unless its specific proposals reach Congress soon.

At the Senate Budget Committee, Sen. William Armstrong (R-Colo.) complained that critics were acting like a fireman who insists on checking the water pressure before turning a hose on a blazing building. "What we want to know is whether we're throwing water or gasoline on it," responded Sen. Ernest F. Hollings (D-S.C.).