By a 2-to-1 majority, Americans say they want Congress to make substantial changes in President Reagan's proposed budget, according to a new Washington Post-ABC News poll.

The public is more pessimistic about the nation's economy than at any time since Reagan's election, the poll shows. Citizens tend to reject Reagan's overall call for tax cuts and decreased spending on social programs, but they are split on the specifics of these proposals.

One recommendation that appears to be gaining in public support is the deferral or elimination of one or both of the 10 percent income tax decreases scheduled this year and next.

In addition, the poll shows, skepticism about Reagan's fairness continues to harden. More than four citizens in 10 express doubt that Reagan believes that his program will help the economy. Instead, that sizable group says, the president cares more about cutting taxes for the wealthy and eliminating social programs for the needy than he does about improving the economy.

However, the poll finds widespread discontent not only with Reagan's handling of the economy but with that of the Democrats in Congress. Only one-quarter of the public sees the Democrats as providing better alternatives to the nation's economic problems than does Reagan, and a majority says the Democrats are offering either worse alternatives or no alternatives at all.

The survey of 1,672 respondents was conducted by telephone, beginning last Wednesday and lasting through Monday evening. One of the sharpest findings is the increasing polarization of attitudes toward the president and his programs by sex, age and income.

On most issues, for example, women disapprove of Reagan far more than men do. The 51 percent of the population with annual household incomes of less than $20,000 consistently disagree with Reagan and his policies, but the president enjoys strong support from those with annual incomes of more than $30,000. Elderly Americans are more critical of Reagan than younger ones are.

In the most extreme example of polarization, blacks are overwhelmingly opposed to Reagan, while whites tend to be divided equally on many issues.

Such jarring views of Reagan could be politically telling this fall. Despite the conviction that the Democrats are not offering better ideas than Reagan's to aid the economy, the public, by a 55-to-36 majority, now prefers Democratic candidates to Republicans in the November congressional elections.

In all, 62 percent of those interviewed say Congress should make substantial changes in Reagan's budget proposals.

Forty-six percent say one or both of the two tax cuts scheduled to take effect in the next 16 months should be delayed, but 43 percent want the tax cuts on schedule. The split is even narrower over elimination of one or both of the tax cuts: 44 percent favor eliminating at least one of the cuts, 45 percent want both to take effect.

Nevertheless, those figures appear to represent considerable change among the electorate over a short period of time. Most polls, including one by The Post and ABC News three weeks earlier, showed the public almost unanimously opposed to any tax increase.

Polls are often described as snapshots of a moment in time, with little ability to predict future sentiment. Signs of economic recovery no doubt would increase Reagan's popularity. But in this snapshot one part of the picture looks blacker for Reagan than any other.

"I'm going to read two statements," Post-ABC interviewers said. "Please tell me which of the two comes closer to your own view:

"A. Reagan really cares more about cutting taxes for the wealthy and eliminating social programs than he does about helping the nation's economy.

"B. Reagan believes strongly that his cuts in spending for social programs and cuts in taxes will help the nation's economy."

A majority of 52 percent agreed with Statement B. But 43 percent chose Statement A, a formulation that must be viewed as a firm distrust of the president and his priorities.