CAMPAIGNS have their effect on a candidate. This one certainly did on this President. Thus, 1984 has, indeed, been a big learning year for Mr. Reagan. * First and foremost, what has been widely perceived as a ''thaw'' in the Reagan portrayal of the Soviets was shaped to a large extent by the persistent effort of the Democrats to portray him as a belligerent President.
Gen. Brent Scowcroft, a leading foreign affairs adviser to the Reagan administration, told reporters the other morning that it was his impression that ''the President was genuinely concerned about comments that he was in fact a warmonger. I think he felt his way was the way one had to deal with the Soviets.'' Scowcroft then went on to say he thought that, startled by this public perception of him, Mr. Reagan did soften his rhetoric.
And it was obvious during the campaign that the President stepped up his efforts to gain an arms control pact with the Soviets - efforts that culminated in his talk with Soviet Foreign Minister Andrei Gromyko and, now, with US-Soviet arms talks in January.
* Second, the President is persuaded by election results that the American people are now ready for him to sit down and work out a nuclear pact with the Soviets. In 1982, as Reagan said again and again, he saw a public that believed the US was too militarily weak, vis-a-vis the Soviets, to enter arms negotiation without being at a severe disadvantage.
Now the President feels that the arms buildup - together with a revival of respect for the US from other nations - is such that he is able to go into arms talks on an equal basis with the Soviet leaders.
Reagan also had to learn that there is, indeed, widespread public concern over the deficit - and that the people now expect him, as their President, to put a deficit-reducing package into effect - and soon.
They want him to get this job done, including persuading Congress to go along with him on some type of bipartisan compromise. It will not be enough for the President to blame the Democrats if he comes away empty handed. The voters will hold him and the Republicans responsible if this deficit is not controlled.
Then, of course, the President had to note that some minorities, notably black and Jewish voters, weren't involved in his landslide.
The President, as he said several times during the campaign, believes he has done well by the blacks and the poor by shaping an economy that is cutting into joblessness. And he sees himself as a steadfast friend of Israel who, as such, deserved more from the Jewish voters.
Now the President has attacked the Soviet Union for what he calls its ''systematic anti-Semitic campaign'' and harassment of the Sakharovs and other dissidents. And, after being criticized by black leaders in this country for his ''quiet diplomacy'' approach to South Africa, Mr. Reagan has said that ''We feel a moral responsibility to emphasize our concerns and our grief over the human and spiritual cost of apartheid in South Africa.''
It is only fair to say that the President is taking these positions because he believes it is the right thing to do. But the election had to alert him to the dissatisfaction with him in black and Jewish quarters.
Finally, the President has to see in the results a clear personal mandate for him and for him to stay on course. He had asked Americans if they felt better off than they did four years ago - and the bulk of the voters have given him a resounding ''yes.''
But within that approval vote Mr. Reagan must also see the extremely difficult road ahead for him and his administration: He must keep Americans feeling good, about themselves, their economy, their country. He knows he cannot rest on his laurels.
He certainly can't let the country sink into a recession. He could blame previous Democratic administrations for the one before. But responsibility for another recession would clearly be his - in the eyes of the public. Further, Americans do expect Reagan to move effectively to lessen the dangers of nuclear war. Indeed, Mr. Reagan must continue to make Americans feel good. It's a mighty challenge.