Urging fellow Democrats to abandon the "lazy orthodoxy" of the past, Arizona Gov. Bruce Babbitt called on his party yesterday to endorse the Treasury Department's "superb" tax-simplification proposal.

With President Reagan and key Democratic and Republican leaders expressing only lukewarm interest in the plan outlined on Tuesday by Treasury Secretary Donald T. Regan, Babbitt told a forum on the future of the Democratic Party that the plan is good for individual taxpayers and begins to correct the inequities of corporate taxation.

"It is a superb proposal," Babbitt, a leading spokesman among younger governors, said. "It is so good that the president will not have the courage to endorse it."

To the Democrats he said, "Let's step forward and endorse it. It goes as far as Bradley-Gephardt and a few steps farther."

A proposal sponsored by Sen. Bill Bradley (D-N.J.) and Rep. Richard Gephardt (D-Mo.) is similar to the Treasury's in that it would replace the existing 14 individual income tax brackets with three rates, while eliminating many deductions.

Babbitt said he expected many governors to oppose the Treasury plan, in part because it eliminates the deduction for state and local taxes. But, calling it "a classic federalism proposal," he said Democrats who have been talking about a more equitable tax system ought to respond to their own logic by supporting the Regan plan.

The Arizona governor spoke at a forum sponsored by the Coalition for a Democratic Majority, an organization of Democrats who stand for a strong defense and an internationalist posture on foreign affairs.

Virginia Gov. Charles S. Robb told the forum that the national party has lost favor among southern voters because it has become the party of "too many messages."

"What we must become, if we ever hope to recapture the White House or the Senate or the South, is the party with discipline and with hope," Robb said.

If yesterday's four-hour gathering was a guide, the Democrats won't easily solve the problems that have cost them the White House in four of the last five presidential elections. The tensions between the older, traditional Democrats and such younger, more conservative leaders as Robb and Babbitt was apparent throughout the day as panelists wrestled over how the party could maintain its commitment to the poor and to disenfranchised citizens while trying to balance the budget and become the party of economic growth.

Even clearer were the divisions between blacks and Jews within the party in the wake of Jesse L. Jackson's presidential candidacy. Jackson, who was not there, was criticized more sharply than Reagan was yesterday among this group of Democrats. Former ambassador Max M. Kampelman denounced the party for not repudiating Jackson at the national convention in San Francisco last summer.