Choosing a president has become so long and restrictive a process that many Americans get downright bored with it. Nonetheless, with the presidential season upon us, it is important to remind ourselves that leaders and leadership really do matter. Presidents, prime ministers, first secretaries change history. Deng Xiaoping, Charles de Gaulle, Margaret Thatcher and, yes, Ronald Reagan are examples of recent leaders who have not only affected their countries' policies -- they have changed the very direction of policy.

Post columnist David Broder is no supporter of Ronald Reagan. But Broder recently wrote that, like it or not, Reagan in his seven years as president has turned around entrenched policies in several important domains. Taxes that had been rising since the New Deal have been reduced. The principle of progressive taxation on income -- dominant in the United States since the New Deal -- is increasingly giving way to flat-rate taxes on income and to more emphasis on consumption-based taxation.

That's not all. The seemingly uncontrollable trend toward greater centralization of power in the federal government, Broder noted, has been reversed.

U.S. foreign policy has also felt the impact of seven years of Reagan. The principle has been established that the United States has an irrevocable interest in the struggle for freedom everywhere. This principle, though sometimes bitterly contested, will prove as difficult to reverse as the new, major departures in U.S.-Soviet relations, where Reagan has insisted that under the right circumstances it is possible and desirable to increase contacts and eliminate whole categories of weapons without diminishing U.S. security.

Reagan's impact has also been felt in international fiscal institutions such as the International Monetary Fund and the World Bank, where market strategies that had been ignored for decades have broken the monopoly of statist and collectivist assumptions about how economic growth occurs and how it can be stimulated.

Reagan is by no means the only head of state who in recent times has managed to transform politics and political dialogue in his country. He has peers.

Margaret Thatcher has wrought a similar transformation in Great Britain, reversing trends toward public ownership and control with principles closer to Adam Smith than to Fabian socialism. Privatization and decentralization are replacing socialism as social goals. Personal initiative is replacing state planning as the motor of growth.

Gen. Charles de Gaulle faced very different problems in France: chronic instability and weak government had dogged France for more than a century. Strange ideas had taken root that free government must be weak and strong government could not be free. De Gaulle thought otherwise. With brilliant leadership, he synthesized order and freedom and solved with the constitution of the Fifth Republic the problem that had nagged French culture since its 18th-century revolution.

China's Deng Xiaoping looked at his society and saw bureaucratization, stagnation, a closed roll from which the Cultural Revolution had eliminated creativity. Deng opened Chinese society, restored intelligence and intellectuals to its universities, ripped the ideological blinders off the Cultural Revolution, sent students out into the world to learn and bring home new ideas, and set about decentralizing that great centralized state.

Now the accession of Mikhail Gorbachev has signaled the world that a new leader seeks to bring new directions to Soviet economy and society. It is too soon to be certain that Gorbachev will succeed, but he has already established new directions.

What do these leaders have in common?

First, they confront the principal problems that baffle their societies, and they propose solutions. They do not seek to sweep problems under the rug nor to deny them. Instead they define the problem, articulate it for their fellow citizens and restore thereby a sense that it is possible to deal with the difficulty. Each of these leaders had a vision of the goal his society should seek and the means that were appropriate.

Reagan had a vision of a more vital economy and society and a more confident international role in the service of freedom. He believed the means to these ends were deregulation, devolution, decentralization and an emphasis on liberty.

Thatcher had a similar vision and a somewhat similar recipe. Both rejected utterly the notion that their countries had entered an inevitable decline.

De Gaulle had ''a certain idea of France'' as great and respected. He thought he knew the means to achieve this restoration: strong government, a strong executive and a new constitution.

Gorbachev has a goal described in his book "Perestroika." The goal is perfecting and completing revolution. The means are perestroika (restructuring) and glasnost (opening).

Each of these leaders persists in his or her goal, even though each has been the object of intense criticism and struggles for power. Because their departures require change, they encounter more criticism than would less imaginative or successful leaders.

Each knows how to wait. Each has a certain flexibility that permits compromise when necessary. Each knows how to delegate. Each subordinates personal loyalties to overall goals, disappoints old friends and acquires new collaborators along the way.

Finally, each has great, overriding confidence in his or her vision. Leadership does not always appear when needed. It is not always easy to identify. And that's important to bear in mind during a presidential season.