White House chief of staff Donald T. Regan and his legislative strategist, Dennis Thomas, have come up with what they think is a better idea for the president's annual State of the Union address. They have devised a streamlined, shorter speech in which President Reagan will celebrate his optimistic goals of diminished government and everlasting economic growth, leaving it to his aides and the federal budget to fill in the grim details.

The approach is a political gimmick but one that makes sense from the White House point of view. Reagan is at his best when looking ahead with a vision of America that is firmly rooted in the past. Although the president will be 75 years old next month, he has a young man's vision. The United States remains to him a youthful democracy, full of vibrancy and "reaching for the stars," as he put it in his reelection campaign.

First and foremost, Reagan is a salesman. He is more comfortable pointing out the virtues of the new home or shiny car than warning about termites or the repair bills. As his pollster, Richard Wirthlin, has observed, the president is most effective when he is thematic and stresses the values and verities he shares with his fellow citizens. These citizens may be skeptical of some of Reagan's most cherished policies and understand that he does not know enough about the particulars. But they trust him and rate him highly as a leader.

Reagan will contend in the State of the Union message Jan. 28 that family budgets will be served by the massive cuts in the federal budget required by the Gramm-Rudman-Hollings balanced-budget law. "If there's pain associated with the budget cuts, there's more likely to be public support, if there's understanding that the whole is more than the sum of the parts," Thomas said.

Maybe so. But there are also likely to be a goodly number of Americans otherwise supportive of Reagan who will pay more attention to the "parts" of the budget cuts that hurt them most. Reagan doesn't plan to say much about this in his State of the Union speech.

Reagan will not tell older Americans, for instance, that their hospital stays may be reduced because the government will not pay hospitals as much for Medicare. He will not tell Amtrak users that they should fly, drive or take the bus. He will not tell entrepreneurs who need a loan that the Small Business Administration won't be around to give it to them. He will convey the good news, while the budget brings the bad.

What Gramm-Rudman-Hollings is doing may be necessary to shrink the size of government and maintain the economic recovery. Some of it may even be desirable. But very little of it is likely to be accepted by a Congress from a president who won't even consider a tax increase, particularly when Congress has the alternative of doing nothing and letting Gramm-Rudman-Hollings automatically return the president's vaunted defense budget to ground-zero.

Reagan can sell almost anything. But there is a potential downside to selling nothing, as the 1984 reelection campaign demonstrated. While the president's happy talk easily prevailed over the gloomy prophecies of the opposition, he emerged from the campaign with a mandate for maintenance rather than change. As a result, Reagan found it exceedingly difficult to drum up public support for his tax-overhaul plan, the principal item on his 1985 domestic agenda.

A desperate personal appeal in December and lots of Democratic assistance kept the tax plan alive for at least another year. But the experience suggested that Reagan had wasted the opportunity presented him by the 1984 campaign to define his agenda and build a coalition that will survive his presidency.

He has another chance now. Reagan's popularity is at near-record levels, and second-term presidents are more apt to succeed when they invest their popularity rather than conserve it. In the early years of his presidency, Reagan carried the day on Capitol Hill because he bluntly told Congress and the country where he wanted to go. Under the pressure of Gramm-Rudman-Hollings, Reagan may find out this year that something more is needed than happy talk.

Reaganism of the Week: Asked at a picture-taking session in the Oval Office Tuesday whether the Gramm-Rudman-Hollings budget cuts would be too deep, Reagan said: "I never consider any budget cut bad."