Public confidence in President Reagan's ability to control the federal deficit has deteriorated and more Americans trust Democrats in Congress to do a better job than Reagan in handling the problem, according to the latest Washington Post-ABC News Poll.

The nationwide survey, completed this week, found that 50 percent of those questioned believe Democrats in Congress can be trusted to do a better job in paring the deficit, compared with 40 percent for Reagan. Two years ago, the last time the question was asked, Americans were more evenly divided, with 44 percent saying Reagan would do the better job and 42 percent saying the Democrats could.

The findings run counter to recent claims from the White House and Republican strategists that Reagan is more highly regarded than the Democrats on handling the deficit. The president has recently returned to the deficit issue as a political tactic, launching a new round of attacks blaming Congress for the red ink.

The survey shows that Americans by a narrow margin assign more blame to Reagan than to Congress for the deficit, expected to be about $170 billion this year. Of those questioned, 46 percent said they blame Reagan for the deficits, compared with 41 percent for Congress. This is a marked change from earlier in Reagan's presidency when the poll indicated that Congress was blamed more widely than the president for the deficits.

In the new survey, Americans said by a 2-to-1 ratio that they disapprove of Reagan's handling of the deficit. This year they have given Reagan the lowest approval rating in four years on this question, and they have also given him lower marks for his handling of the economy than at any time since 1983.

The findings come as Reagan readies a key speech at the Jefferson Memorial on Friday announcing his proposal for an "Economic Bill of Rights," which is to be a centerpiece of his renewed campaign against Congress on spending issues. He has been trying to recover from the political damage of the Iran-contra affair by giving speeches around the country criticizing Congress and the Democrats on the deficit. Reagan, who initially promised to balance the budget but instead presided over a doubling of the national debt, has said he will not compromise his priorities of increased defense spending, cuts in domestic programs and no tax increases.

White House officials have said they decided to undertake the latest blitz because surveys by presidential pollster Richard Wirthlin and others showed that Reagan still had support for his assertion that Congress has been unable to handle the deficit problem.

The new survey suggests that Americans back some elements of the Democrats' approach to the deficit, including cuts in military spending, but that Reagan has strong support for his demand that income taxes not be increased.

The Democratic-controlled Congress recently voted a $1 trillion budget that included $19.3 billion in new unspecified tax increases, and congressional leaders have been discussing possible boosts in excise taxes.

The new poll found strong support for "sin tax" increases on liquor, beer and cigarettes, and for higher taxes on upper-income people and corporations. However, those questioned overwhelmingly rejected any increase in individual income taxes, gasoline taxes and taxes on telephone usage. The poll showed Americans skeptical over the wisdom of taxing airline tickets to reduce the deficit. These views are largely unchanged from previous years.

The new survey also found that by 2 to 1 Americans would rather cut military spending than raise taxes or cut domestic social programs. By similar margins they would rather cut defense spending than allow the deficit to get bigger. These findings run parallel to other polls showing that Reagan has largely lost the sizable consensus that existed in the early 1980s for a major military spending increase.

The president has recently resumed his appeals for line-item veto authority, allowing him to eliminate specific items in spending bills he does not like rather than vetoing the entire measure. Of those questioned in the new survey, 52 percent said they opposed Congress giving this authority to him, compared with 47 percent who favored it.

The new survey also suggests that Americans are growing increasingly pessimistic about the course of the economy. When asked whether the economy was getting better or worse, only 19 percent said better, the lowest level since the last recession bottomed out in 1982-83, while 38 percent said the economy is getting worse, and 43 percent said it is staying about the same.

Asked whether "things in this country are generally going in the right direction" or whether "things have gotten pretty seriously off on the wrong track," 62 percent said the wrong track and 35 percent said the right direction. This was the most pessimistic view offered since the question was first asked in 1982.

Despite that, the president's overall approval rating remains steady at 50 percent. In some areas, such as his handling of foreign affairs, Reagan has registered a slight improvement from the low levels that followed the initial disclosures in the Iran-contra affair.

As in earlier surveys, the new poll showed that nearly two-thirds of those questioned say Reagan is honest, while the same number say in response to a different question that he has not told the truth about the Iran-contra affair.

The survey showed that Americans are assigning an increasing share of the blame for the scandal to Reagan's subordinates. The advisers were blamed by 69 percent of those questioned, compared with 25 percent who said Reagan was more at fault. Four months ago, 57 percent faulted the advisers and 32 percent blamed the president.

The survey also found that Americans believe by 2 to 1 that the Iran-contra committees are more interested in getting at the facts of the scandal than trying to discredit the administration.

The poll of 1,506 people was conducted nationwide by telephone June 25-29. The margin of sampling error was plus or minus 3 percentage points.Staff polling analyst Kenneth E. John contributed to this report.