THE TENSION between President Reagan and Congress has risen sharply over the winter, and the reasons for it aren't going to fade soon. There's more to it than the normal erosion of authority in the second half of a presidential term, and more than the modest Democratic gains last November can explain. A confirmation vote that would earlier have been routine, in the Adelman case, turned into a dramatic test of the administration's strength. The president's proposals for defense spending are clearly going to be cut more than last year, and domestic spending is going to be higher than he wants. That's quite a contrast to the extraordinary obedience that Mr. Reagan extracted from Congress in the first year of his administration.

That obedience began to dissolve in the autumn of 1981, when Congress, having passed the monumental Reagan tax and budget legislation, began to realize that interest rates were not going to drop and the strategy wasn't going to work. But within the past half-year, the foreign factor has been increasingly influential.

Mr. Reagan has been a lucky president. It was sheer luck that nothing happened abroad in his first year to distract attention from his economic program. But things began to happen in his second year. Among them, those that have most directly affected Mr. Reagan's relations with Congress are the fighting in Central America and, on quite a different wave length, the resurgence of the European peace movement.

The demonstrations in Western Europe against nuclear weapons have not only forced Mr. Reagan to spend much more time on disarmament policy. The spirit of those demonstrations has also flowed into this country and strengthened a similar movement here. The nuclear freeze movement, well represented in Congress, is now attempting to impose its position on a president whom it regards as an adversary. It has created at least a strong minority in Congress where other issues--for example, a 10 percent unemployment rate--have not.

In 1981, as the Central America question was put to Congress, it was a matter of safeguarding the region against Cuban and Soviet tampering. On that subject, congressional opinion is endlessly fragmented. But the nature of the question has changed over the past two years. Now it has more to do with Congress' assertion of its institutional power to set limits to presidential policy. On that one, a large majority of both parties hold very similar views. That's why the Boland amendment passed almost unanimously, and why the votes on aid to El Salvador are going to be difficult for the administration.

Rising resistance from Congress does not necessarily mean poorer performance by the administration. On the contrary, in that first year Mr. Reagan's extraordinary power allowed him to make very expensive mistakes with his excessive tax cuts. But now the world is pressing in on him. That halcyon year when he could devote himself mainly to the budget, and wholly on his own terms, is not going to be repeated.