Americans will welcome the change of tone in President Reagan's public posture. His more realistic appraisal of the US economy and strong call for bipartisan cooperation are just what is needed to begin lifting the nation out of its ''ordeal.'' Mr. Reagan, without appearing on the defensive, nonetheless indicates that he knows he will no longer be able to impose his will on Congress but will have to work with it - and compromise - if solutions are to be found. If he follows through on such a commitment, this should not weaken the President's leadership but strengthen it, for the American people above all are looking for practical results.
Substantively, Mr. Reagan did not yield too much in his State of the Union address, but that is to be expected. As a shrewd politician and bargainer, he does not put all his cards on the table at once. He kept many of his favored policies in place, including the tax cuts enacted in 1981 and a substantial increase in defense spending. It can be anticipated, however, that, as the bargaining over next year's federal budget goes on, he will bend considerably more. Indeed Mr. Reagan's broad proposals for holding down the budget deficits are bound to raise questions:
* Spending freeze. The concept of freezing the federal budget, which Mr. Reagan borrowed from Democratic Senator Ernest Hollings, is an enticing one. The President's guns-over-butter version, however, would require cuts in domestic spending programs while leaving out the whole arms-procurement sector, thus undermining the utility of such an approach.
* Defense cuts. That Mr. Reagan has conceded even slightly on his massive military program is encouraging. But his proposed $55 billion reduction in outlays over a period of five years represents only a slight dent in a budget running more than $1.5 trillion. Under his latest proposal, the 1984 defense budget would still provide for a 9 percent real rate of increase, a figure most lawmakers regard as too high, and unnecessary from the standpoint of national security. Mr. Reagan is correct in arguing that current and planned defense spending is not an unduly high proportion of the GNP. The point, however, is that it is the unprecedented rate of increasem which is helping to fuel the massive budget deficits.
* Tax simplicity. Kudos to the President for raising the issue of simplifying the tax code and making it fairer for all Americans. Here is an area for much-needed reform. Yet it seems inconsistent for Mr. Reagan then to advocate, for instance, tuition tax credits to parents who send their children to private schools and also legislation which would exempt from taxes savings accounts for a college education. Would not such changes merely build even more loopholes or ''tax expenditures'' into the system, complicating it further? If the goal is simplification, some better way could be found to provide financial relief for college students.
* Contingency tax. Mr. Reagan's desire to assure the financial community that the federal government will not allow huge deficits to continue into the future is understandable. But enacting a stand-by tax to start in fiscal 1986 hardly gives confidence that the government will act responsibly nowm. What today's Congress enacts, moreover, tomorrow's Congress can undo. The whole exercise may strike many as a cop-out.
Other initiatives, too, now deserve to be debated. Conspicuously missing from Mr. Reagan's message, for instance, was a commitment to the kind of strong jobs program favored by many legislators - a program which is needed and may well make sense once spending in other areas has been brought under control. In any case, the essential point is that the President seems ready not just to castigate government, as he has done for the past two years, but to work with Congress to make government a vehicle for positive action. Indeed Mr. Reagan seemed almost surprised by the standing ovation he elicited in the House chamber when he declared, ''We who are in the government must take the lead in restoring the economy.''
With a more flexible, more pragmatic approach, the President has begun building a bridge to the legislature. Americans can only hope that the bridge gets built.