To the widespread belief that the Democrats have lost both their voice and their sense of direction, there are now at least two important rebuttals. New York Gov. Mario M. Cuomo, the keynoter of last summer's Democratic convention, and Sen. Gary Hart of Colorado, the leading challenger at that convention, have given speeches that clearly suggest that two potential leaders of the Democratic comeback effort have not lost their compasses or their rhetorical command.

Even though they avoided many of the troublesome specifics, there were clear differences in emphasis in the speeches Hart delivered at Boston's Faneuil Hall last month and Cuomo gave a few days later at Yale. But it is significant that these two men, stereotyped as the spokesmen for the "revisionist" and "traditionalist" wings of the party, found common ground with each other -- and for a challenge to President Reagan.

Obviously there can be no cheap consensus within the Democratic Party based on the perceived convergence of two leading figures. The more conservative views of Sen. Sam Nunn of Georgia or Gov. Mark White of Texas, the urban and minority perspectives of Philadelphia's Rep. William Gray and Mayor Wilson Goode, have to be blended into the mix before anyone can claim that the Democrats have their act together.

But the Cuomo and Hart speeches do rebut the notion that the Democrats as a species are still reeling in shock from last November's presidential shellacking and don't know which way to turn.

There are four principles on which Hart and Cuomo clearly see eye-to-eye. The first is the belief that Reagan's policies endanger the long-term economic health and social stability of this society. If events in the next four years prove this view incorrect, then the Democrats' prospects of a 1988 comeback will indeed look bleak.

But if the economy sours as a result of soaring deficits and the nation polarizes between haves and have-nots, these two Democrats, at least, will be in a position to say that even when Reagan was riding high, they told us what lay ahead.

Their rejection of Reaganism extends beyond words to deeds. Cuomo last week challenged the president to his face, in a White House colloquy on the budget. Hart, who has consistently voted against Reaganomics, also voted against entrusting the leadership of the Justice Department to Edwin Meese III, who echoes Reagan's views on social policy and constitutional is- sues. On all these fronts, the lines are drawn. The second point on which Hart and Cuomo agree is that the values for which the Democratic Party has fought for 50 years remain valid. "Despite the events of last November, I haven't changed the underlying tenets of my political philosophy," Cuomo said. "Why should I seek new principles?"

Not surprising from the governor who sits in Albany where FDR once sat? Perhaps not, but Hart voiced the same view. Though saying "we are participants in an era of profound transition," he reaffirmed "one of the purest old ideas," the belief that "society as a whole, through our government, has a moral responsibility . . . to achieve real justice . . . and assure the basic necessities of life."

The third point of agreement is that government must conceive itself as a partner in furthering the powerful forces of economic change and growth, not frustrating them. Predictably, Hart emphasized the need for policies that "encourage improved productivity" to meet competition in the international marketplace. That is the ideology of a high-tech, growth-state politician.

But Cuomo, a "rust-belt" governor, was equally clear in saying that "programs and policies change," and boasting that his administration's "new ideas" include "new centers for innovation that will spur the growth of high-tech in the state."

Finally, both Hart and Cuomo explicitly recognize that the test of credibility for the Democratic Party is the willingness of its leaders to put the national interest above any constituency interest. Hart gave evidence of his seriousness by introducing legislation for "a new system of national service," both military and civilian, that would "ask young Americans" -- among whom he had his greatest support in 1984 -- "to return some of the advantages and investments they have received from our society."

Cuomo demonstrated his grasp of the same point in a more dramatic way. He told his labor-union backers -- without whom he would not have been nominated or elected in 1982, he acknowledges -- that he would not support their candidate for the chairmanship of the Democratic National Committee, or even the disputed right of the at-large labor members of that committee to vote in the contest last month.

Two speeches -- and two men -- do not a consensus make. But they are a start and, for Democrats, a welcome sign that all may not be lost.