Harry C. McPherson Jr., a former aide to President Lyndon B. Johnson and now a Washington lawyer, last week spoke for a lot of Democrats in the wake of the pasting they took at the hands of President Reagan, but he added a sort of cockeyed caveat born of years of experience with the Democratic Party.

"I feel dumbfounded and pessimistic, and I can't say what to do to bring the Democratic Party together," McPherson confessed to a luncheon seminar of the Center for National Policy, a Democratic group. "But life does send up flowers through the manure pile."

Gov. Bruce Babbitt of Arizona cast the loss -- the fourth the party has suffered in the past five elections -- in starker terms.

"One more crash like this will be the prelude to the great reapportionment election of 1990," he warned the Coalition for a Democratic Majority at another election post-mortem recently, expressing the Democrats' apprehension that the Republicans may be on their way to becoming the majority party.

The Democratic Party has a 34-to-16 lead over the Republicans in governors, controls the House and two-thirds of the state legislative chambers, and hopes to recapture the Senate in 1986.

But the party that has gloried in what Franklin D. Roosevelt made it, a national, presidential party, is suffering an identity crisis because it is a loose confederation of state and local parties, apparently incapable of uniting nationally to win the White House.

"The Republicans have reluctantly accepted the New Deal, but they keep chipping around the edges of what we've done, which is why we do well in congressional races," said Stuart Eizenstat, a top adviser to former president Jimmy Carter. "But we're a minority at the presidential level, which is where Americans express their basic values."

The Democrats have identified a number of reasons that they have arrived at this state, most of which they believe reflect the fact that they are prisoners of the past. Various Democrats contend that:

* Social and demographic changes have greatly altered the New Deal coalition, and the national party leadership, failing to respond to this, is on the wrong side of the changes.

"The party is in transition because the country is in transition," said Richard Moe, a longtime adviser to Walter F. Mondale. "The Republicans understand this better and they're setting the national political agenda."

* The Republicans are perceived as the party of growth and opportunity while the Democrats are perceived as the party of the past and status quo, concerned with preserving the welfare state they created.

"We have no post-welfare program," Eizenstat said.

* They are caught in the bind of their tradition of being the party of minorities and the poor and the apparent growing impatience of the post-New Deal generation with welfare programs.

"It kills me to have to ask if a party so identified with the problems of the poor and minorities can win in the 1980s," said Hyman Bookbinder of the American Jewish Committee, speaking as a lifelong party activist and former official of the Great Society's War on Poverty.

* The Democratic Party also is a captive of its role in the Vietnam war and the trauma of the Iranian hostage crisis, and is perceived as being soft on national security and foreign policy.

"Why can't we meet the threshold test of strength in defense policy?" Moe asked. "We're still torn apart by Vietnam and Iran."

* It also is perceived as a captive of its constituent groups, such as organized labor, the teachers' unions and every minority caucus from blacks and Hispanics to homosexuals.

* Since 1968, when the Democratic Party was traumatically torn at its national convention in Chicago by dissent over Vietnam and reformers rebelling against the party's bosses, it has been perceived as being more concerned with internal party procedures and compulsive tinkering with its presidential delegate selection rules so that everyone is included than with issues of concern to the average citizen.

"We're obsessed with our process, we're compulsive tinkerers, because there's such a vacuum on policy," Babbitt said. "The reason interest groups are so predominant is because the mainstream candidates aren't saying anything."

* Whites, particularly white males, are deserting the party, and in the South the parties are becoming racially polarized between white Republicans and black Democrats.

"Blacks own the Democratic Party," McPherson said. "Wasn't the role of Jesse Jackson in North Carolina the reason Sen. Jesse Helms won? White Protestant male Democrats are becoming an endangered species."

This is an impressive and undoubtedly incomplete list of problems and complaints, but some Democrats such as Moe and Brian Atwood, executive director of the Democratic Senatorial Campaign Committee, say the solution could be comparatively simple.

"It will be impossible to get this disparate party to agree on anything until we get a powerful, popular figure at the top of the ticket," Atwood said. "In the last three weeks of the campaign Mondale created a boon for candidates in the Northeast and Chicago and a disaster in the South. Because he knew he would lose, he went back to the issues he came into politics with years ago."

The years have changed the constituencies and the appeal of these issues, Eizenstat said.

"Economic and demographic changes have shattered the New Deal coalition," he said. "The growth of postwar incomes put 40 percent of the population in the $25,000 to $50,000 annual income range. We get the votes of most whose incomes are under $10,000, but there are fewer of them and their turnout is low.

"In the 1950s a third of the labor force was union. Now it's one-fifth, and in the Sun Belt it's a tenth. People are moving from the Snow Belt to the Sun Belt -- by the year 2000, Texas, Florida and California alone could have 40 percent of the electoral vote -- and adopting the conservatism of their neighbors, self-help and neighborhood help and a suspicion of large institutions. Populist Republicans are taking on the traditional anti-establishmentism of the Democrats, and we're on the wrong side of the social changes."

Ben W. Heineman Jr., an official in the Carter administration and now a Washington lawyer, told the Center for National Policy that the Democratic Party has to give precedence to economic policy and "become the party of growth, productivity, opportunity and hope" rather than the party of social policy -- "advancing entitlement group coverage, health, welfare and unemployment compensation."

He contended that one problem this year was that Mondale, Rep. Geraldine A. Ferraro (D-N.Y.), Jackson, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and the leaders of the AFL-CIO are social policy Democrats.

Bookbinder countered that separating economic and social policy isn't that easy, and former Sen. Edmund S. Muskie, chairman of the Center for National Policy, noted that many of the problems addressed by the Great Society still were unsolved.

"New ideas aren't the answer. Creating a Department of Education or moving the U.S. Embassy in Israel or changing the proportion of presidential delegates aren't the test," said Ted Van Dyk, a founder of the center. "Most people vote on where they think the party stands on big issues, not fragments."