At the magic moment when the television networks decide that they have had enough of vote-counting and project the winner of the presidential election, Ronald Wilson Reagan is likely to be at the zenith of his power and influence.

If Reagan wins big, especially with coattails, he will have opportunities unequaled since Lyndon B. Johnson's to put his indelible stamp on national policy, and no war to stop him from doing it. Some of these opportunities may be remedies for the most conspicuous failures of his first term in dealing with the deficit and the Soviet Union, but they are opportunities nonetheless.

The president's strategists are convinced that he can take advantage of what they see as a "window of opportunity" for him in early 1985, when his personal popularity and a renewed mandate promise a second honeymoon with Congress.

Even the rosiest optimists in the White House do not expect the honeymoon to weather an economic downturn or another upheaval in Central America. And however long the honeymoon, Republican control of the Senate is unlikely to extend beyond 1986, when 22 GOP senators but only 12 Democrats face reelection.

This means that the political clock will be ticking from the moment of reelection. It will be ticking on the deficit, an issue on which the president and Congress must act before a new recession occurs. It will be ticking on arms control, where the Soviets inevitably will be tempted to wait out Reagan the longer his administration delays in getting its act together.

Further down the trail, there also are potential conflicts between disparate elements of the Reagan coalition. Like the New Deal coalition of Franklin D. Roosevelt, which included southern segregationists and minorities of every coloration, the Reagan coalition embraces zealots who would use the power of the state to prohibit abortion and require school prayer, and independent, prosperous younger voters who resent intrusion into their lives as much as they do the welfare state.

Reagan is no Roosevelt, but he shares with his first political idol a commanding presence that tends to obscure differences. Even without the 22nd Amendment, Reagan would be a lame duck because he would be nearly 78 at the end of a second term. Because the Democrats don't have to face him again, they can run the risk of cooperating and deservedly sharing in the credit if he succeeds.

What is required of Reagan, in addition to the graciousness that can be expected of him in victory, is a signal that he has a second-term agenda and the willingness to implement it. For starters, he might install some acknowledged senior negotiator, of the sort epitomized by Gerald R. Ford's former national security affairs adviser, Brent Scowcroft, to oversee and manage the disorderly arms-control process that was a feature of Reagan's first term. He also might place some new and energetic people in the Cabinet.

One sign that would be helpful but may be difficult for Reagan to give so soon after his happy-days campaign is frank and public recognition that the federal deficit is the problem Reagan recognized it to be when it was much smaller and the property of Democratic presidents.

Does Reagan have the political will to do all or any of this? Some, even in his administration, doubt it. Reagan does not easily admit imperfections in policy positions or appointments, and he persistently has overrated his Cabinet's competence.

Despite the president's vigorous windup campaign, which took him to 10 states in five days, there are those who question his energy level. He will make two trips, totaling 10 days, to his ranch in November.

But hardly anyone would begrudge Reagan a vacation. What matters is not that he retreats to his Santa Barbara ranch for a breathing spell but what he does when he returns. People like this president and give him a lot of running room. It will be up to Reagan to demonstrate that he has an agenda and the will to accomplish it, as well as a rosy view about the future of America.

Reaganism of the Week: Asked in Philadelphia last Monday about challenger Walter F. Mondale's comment that the president had demonstrated overconfidence by staying at Camp David, Md., over the weekend, Reagan replied: "Well, if I hadn't been overnight at Camp David, I would have been overnight at the White House, and either place is good sleeping."