GARY Hart has probably sensed more accurately than his rivals in the Democratic Party that there are a number of things that most Americans approve in the policies of Ronald Reagan and that they want to keep those things. Senator Hart has shaped his own political stance in accordance with his perception. He is not running against the best of the Reagan record.
This makes for a new problem for President Reagan and his advisers. They expected to be running for reelection against Walter Mondale. They were all set to aim at the political protege of Hubert Humphrey, the vice-president to Jimmy Carter, and the declared champion of organized labor, who repeatedly refused early in the campaign to name a single point on which he differed from the AFL-CIO.
The strategy against a Mondale candidacy was easy. Mr. Reagan could brush it off as just another rerun for the old ''liberal'' formula of ''tax and spend.'' He was preparing to run against those several features on the Democratic Party's record which have been exhausted or discarded by time but which Mr. Mondale can never shed entirely. He too may regard them as obsolete. But he was so intimately associated with them over the years that he is inevitably associated with them in the public mind.
A Hart candidacy has emerged which makes for an entirely different targeting problem for the Republicans. Senator Hart has dissociated himself from organized labor. He is against overregulation of the economy. He favors competition. He opposes trade protectionism. He is for a strong defense posture.
In other words, he recognizes that the tax-cutting and welfare-curbing features of Reaganomics are associated in the public mind with economic recovery and reduction in the inflation rate. They are accepted as good and to be retained. The American people of 1984 have rejected unlimited welfarism and rising taxation. So has Mr. Hart. And he can do so convincingly because he has not been associated with those features of old Democratic orthodoxy.
No single set of political and economic policies is good forever. The problems of a nation change from age to age. The change calls for revisions of the old formulas. The FDR-Harry Truman-New Deal formulas worked for a remarkably long time, in fact down into the Carter presidency.
But by the Carter years the old remedies had run out of steam. They did not prevent the inflation, and the recession which came after it. We tend to label Mr. Reagan a ''conservative.'' He has in fact been a radical reformer. He engineered a radical break from the past. He reorganized and re-formed the American economic system.
But in some respects he ''overshot his target.'' He carried deregulation to the point where, under James Watt, the environmentalist community rose in rebellion. Basically, Mr. Watt had to go for carelessness about forests and streams. Anne Gorsuch Burford had to leave for the same fault.
In foreign policy Mr. Reagan was on politically safe ground when he ''liberated'' Grenada. Almost everyone applauded. But it became a different story in Lebanon, when one terrorist bomb took more than 200 US marines.
Rebuilding the American armed forces is still a generally popular cause, but when it means a $500 billion deficit and the danger of a return of high inflation, people begin to ask questions, and to think that perhaps Mr. Reagan has overdone weapons building just as the Democrats once overdid welfare.
The interesting new feature on the American political scene is that Mr. Reagan may not be able to run against the Democratic Party's flawed past, as he had expected. Instead he may be up against a new-style Democrat who will treat Mr. Reagan himself as belonging more to the past than to the future.
The immediate effect is that the Republicans have to plan two quite different strategies for the final campaign. They cannot know yet whether they will be running against Mr. Mondale and the old Democrats or Senator Hart and the new Democrats. Both strategy and tactics have to be different if Senator Hart should happen to be the Democratic contender.