It's still ''Mondale vs. Hart'' in the headlines. But behind the scenes, the Mondale campaign has begun gearing up for ''Mondale vs. Reagan.'' The Mondale team wanted to take aim at the Reagan White House months ago. But Gary Hart's surprising strength and his wide appeal to young Democrats diverted Walter Mondale from his prime 1984 target.
That wasn't all bad, the former vice-president's aides concede. Senator Hart's tough opposition uncovered weaknesses in Mr. Mondale's strategy. He made Mondale a better campaigner.
Mondale's easy victory over the weekend in the Texas caucuses, where he trotted away with a 5 to 3 margin over Hart, leaves him free to turn more and more of his attention toward Mr. Reagan.
In the past two weeks, Mondale has taken the first steps toward sharpening his campaign themes for the fall. Voters can expect to hear these arguments over and over again from now until the November election. Among the major ones revealed so far:
* Reagan has substituted an arms race for arms control. ''In the year 2000, unless we change course, we will be living in perpetual fear,'' Mondale told an audience at the University of Cincinnati.
* Reagan has ''put human rights on the back burner. In the year 2000, we will still be haunted by the ghosts of those we failed to help - from Central America to the Philippines.''
* Reagan has shredded environmental regulations. ''He has endangered our health and our safety by failing to enforce (environmental) laws.'' The number of federal cases being brought against polluters is down 65 percent, Mondale told a crowd at South Bend, Ind.
* Reagan's ''Star Wars'' proposal for an antimissile defense system would ''open the heavens for warfare.'' This proposal is a ''critical turning in the arms race.''
* Reagan has failed to set high standards of conduct for those in his White House. ''There's a long line of officials in this administration who've been mired in controversy over their legal and ethical behavior.''
* Reagan has ''savaged'' federal aid to education. This could lead to severe shortages of skilled workers for American business in coming years. ''For three years, Mr. Reagan hasn't lifted a finger for education, except to point to it in blame,'' Mondale told an audience at San Antonio, Texas.
* Reagan has retreated from civil rights and women's rights. ''This administration has attempted to dilute the Voting Rights Act, which is, in many ways, the most vital and effective federal civil rights law.''
* Reagan has been irresponsible with the budget. ''He has saddled America . . . with the biggest debt in world history.''
In three or four additional speeches during the next month, Mondale plans to expand on these and other criticisms of Reagan. These themes, his staff argues, are ones that the grueling campaign with Hart has shown play well with 1984 political audiences.
Throughout all these speeches, Mondale is emphasizing that he has a vision for the future of America - a concept that has been the backbone of the Hart campaign.
Democratic pollsters have found that while Reagan is credited with strong leadership, voters don't consider him a ''man of the future.''
The public, Democrats say, is yearning for someone who can grapple with such problems as the future of the arms race, and future industrial policy to meet foreign competition from such countries as Japan and West Germany.
Further, the aides say that by emphasizing arguments that were used by Hart as well as John Glenn and other Democratic candidates, Mondale hopes to heal party divisions prior to the national convention in July.
The latest Mondale strategy may also help to overcome what pollsters say is a weakness of his own campaign. Most voters, pollsters say, consider Mondale a man of the past, a representative of old-time Democratic politics from the Johnson-Humphrey era of the 1960s.