President Reagan took center stage last week for the first time since his reelection and dealt with the most emotional and important issues of his presidency in ways that offer valuable clues about his probable conduct during the second term.

On abortion, unquestionably the most emotional issue, Reagan spoke to more than 70,000 demonstrators who braved the Washington cold. He seemed to offer the aid and comfort he has long promised to the antiabortion cause.

"I'm proud to stand with you in the long march for the right to life," Reagan told the cheering throng.

But the metaphor was not totally appropriate. Reagan was sitting in the Oval Office and speaking through an electronic hookup to a crowd that could not see him.

Nor could the nationwide audience of television viewers see Reagan that evening on network news shows or in photographs in morning newspapers the next day. Strict instructions had been given that no photo was to be allowed of the meeting Reagan later held with leaders of the antiabortion movement.

It seemed strange conduct for a White House that lives by the "photo opportunity."

But Reagan usually has been adept at protecting himself from his friends as well as his enemies. He has given ideological supporters verbal sustenance on social issues without doing much to translate their pieties into law.

Reagan has maintained a discreet distance even from causes that mean the most to him. In so doing, he subtly reminds Americans suspicious of causes and ideologies that he is not a zealot.

Reagan, who as governor of California signed what was then the most permissive abortion law in the nation, has for four years shrewdly avoided commitment to a particular antiabortion amendment, citing conflicts within the antiabortion movement as his excuse. He continues to hold this position, despite saying to one antiabortion leader that he favors the most sweeping of the amendments.

What was interesting about Reagan's latest distancing act was that it was the kind of political conduct that a few months ago would have been ascribed to the reelection campaign.

But Reagan will never face election again. Last week's speech and the subsequent refusal to pose with leaders of the movement suggest a basic pattern of political conduct that goes beyond campaign requirements.

The pattern is to give supporters much of what they have been promised but less than they need. Reagan maintains his identity and his freedom to manuever by always saving something for himself.

The most important issue of Reagan's second term, the one on which history is likely to judge him, is arms control. Reagan is operating on the premise with which he began his presidency: that the Soviet Union will come to the bargaining table with serious proposals when it recognizes that the United States is committed to a long-term defense buildup.

But Reagan has become highly sensitive to the charge that his lack of knowledge about nuclear weapons and his failure to involve himself in the arms control process contributed to the breakdown of U.S.-Soviet arms talks during his first term.

So last week, as the administration's new arms control team assembled for a symbolic first meeting, Reagan made a point of presiding and instructing the negotiators to do their homework.

In case reporters missed the message, there were plenty of administration officials around to tell us that the president was becoming involved in the arms control process. There was also a photo opportunity with the arms control team -- on the same day that Reagan refused to allow one with the antiabortion leaders.

Publicly, of course, Reagan has never conceded that he wasn't involved in the last set of arms negotiations. But there is widespread agreement among those close to the process that he participated more fully in the meetings leading to the Geneva agreement to renew arms control than he had before.

While the present ballyhoo about the president's participation can be viewed as a backhanded admission that there was a missing force at the top in the first term, it is nonetheless a welcome and necessary change.

"The president has found that personal involvement works," an aide said last week.

The cynical view might be that it is about time. But it is an encouraging omen for the second term. Reaganism of the Week:

Speaking to a meeting of Western Hemisphere legislators in the Oval Office on Thursday, Reagan said, "There's never been a war between two free countries."