Warned by one of their leading economic advisers that President Reagan's program could look pretty good at election time next year, top officials of the Democratic Party struggled today to find new policies that could fuel a political comeback.

Along the way, they stumbled into some of their old divisions over busing, welfare and affirmative action, as New York Mayor Edward Koch provoked angry responses from black officials and some liberals to his barbed attacks on those programs.

The first meeting of the Democratic National Strategy Council of federal, state and local elected officials produced no clear consensus on an alternative to Reaganomics.

While former vice president Walter F. Mondale said Reagan's mixture of tax cuts and budget reductions is "crazy" and won't work, Walter W. Heller, former chairman of the Council of Economic Advisers, said that might not be at all obvious a year from now.

Heller said the president's economic program is laden with contradictions, but warned the Democrats "not to delude ourselves into thinking Reaganomics will self-destruct politically."

On the contrary, he said, by next fall, Reagan "might make a plausible case" for success. At that time, he predicted, taxes will be down 15 percent, interest rates will be reduced "substantially," inflation will be lowered by 30 or 40 percent and the economy will be on the way back up from the slump that is now under way.

"I conclude," said Rep. Gillis W. Long (D-La.), "that you think we may be running next year when the American people think it's a great success?"

"It could be made to look pretty good," Heller said, adding that most of the costs of the Reagan program should be more obvious by 1984.

Mondale told a news conference later that if the economy prospers the Democrats will fight the 1982 campaign "on other issues. This party cannot appear to want to win off the heartache of the American people," he said.

Mondale was the final speaker at a two-hour session this morning, when most of the 55 Democratic officials offered their own, widely disparate prescriptions for the future.

He said he found consensus on the importance of economic growth, social justice, environmental protection, energy self-sufficiency, strong defense and control of nuclear weapons.

Those same themes were emphasized in party Chairman Charles T. Manatt's closing statement, which promised that task forces of the 100-member council would provide "new strategies for hope" to replace the Republicans' "old ideas."

The faint outline of a possible Democratic economic alternative appeared in the afternoon discussion, led by Reps. Timothy E. Wirth of Colorado and Richard A. Gephardt of Missouri. It envisages increased investment in education, research and technology, targeted tax cuts and much more direct involvement of government with business and labor in shaping long-term investment decisions.

California Gov. Edmund G. (Jerry) Brown Jr., promoting a similar approach, said the policies of Franklin D. Roosevelt could serve the Democrats as a guide. "The creation of private wealth is inextricably linked to public investment," he said, "and the Reagan program ignores that fundamental fact."

But even as vaguely described, the new economics drew expressions of misgivings from Democrats as diverse as Detroit Mayor Coleman Young and Rep. Dante B. Fascell of Miami. Both said the Democrats might wind up representing only the losing constituencies and regions in the kind of economic transition such a plan would speed.

Comments in the six hours of round-tables that began Friday night showed how far the Democrats are from agreement on any of the major issues facing the country. Most of the debate was polite in tone, but Koch stirred an angry reaction with his remarks.

Koch, who has been nominated by both the Democrats and the Republicans for reelection next month, said too many federal programs had been run "ass-backwards," and said that Democrats were defeated last year "because we were out of step with the country" on such issues as busing, welfare and job quotas.

State Sen. Clarence Mitchell III of Baltimore, shouting, said that if the Democrats try to "out-conservative the conservatives, then the Democratic Party is doomed."

Outside the hall, Koch had a roughly worded exchange with Rep. Charles B. Rangel of New York, who told him he should not offer himself "as an example to the national party when you lost all the Hispanic and black precincts" in the recent New York Democratic primary.

While the New Yorkers were feuding, Sun Belt representatives such as William V. (Bill) Alexander Jr. of Arkansas and Wes Watkins of Oklahoma asked their colleagues to "learn to count," and to make room in the party for conservative Democrats from the growth areas.