In victory and defeat last week on different issues, President Reagan demonstrated that he is willing to set aside cherished misconceptions when it comes time to strike a political deal.

Reagan's most remarkable about-face was on acid rain, where his record is widely perceived as disgraceful. While acid rain has steadily destroyed lakes and fisheries in Canada and the Northeast, Reagan has persistently resisted entreaties by Canadian and U.S. officials to do something about the problem.

Reinforced by the penny-wise and pound-foolish recommendations of then budget director David A. Stockman, who thought that federal spending on acid rain was a boondoggle, Reagan largely ignored the issue during his first term.

After Stockman departed, obstacles remained in Reagan's head, where borderline scientific notions drift like pollutants across the Canadian border. Reagan believes that natural processes, particularly forest fires, are the cause of acid rain. His view is reminiscent of crucial environmental discoveries during the 1980 campaign when he inisisted that trees were the primary source of air pollution and said that Mt. St. Helens had spewed out more pollutants than all of the world's automobiles.

On the acid-rain issue, Reagan proved impervious to scientific evidence and was unimpressed by findings of a year-long joint U.S.-Canadian study that man-made pollutants "contribute far more to acid rain in the United States than natural sources." But there is nothing quirky about Reagan's grasp of political reality. When he learned that his friend and fellow conservative, Canadian Prime Minister Brian Mulroney, needed U.S. action to survive politically, the president endorsed the study and committed $2.5 billion in U.S. funds to develop technologies for combating acid rain.

Reagan displayed similar realism in endorsing a $250 million economic aid program for Northern Ireland. This useful handout is designed to sugarcoat the Anglo-Irish Accord, which gives the Republic of Ireland a voice in Northern Ireland's affairs and will help protect the civil liberties of the Roman Catholic minority. But it is ironic that most of the aid is likely to be spent on precisely the type of urban development and housing projects that the Reagan administration has been systematically eliminating in this country.

Even on the issue of aiding the Nicaraguan contras, which reveals Reagan at his most ideological and apocalyptic, the president is willing to play the practical politician. While White House communications director Patrick J. Buchanan was beating up on Democrats, administration strategists were cutting deals in advance of an expected second House vote next month. White House legislative strategist Dennis Thomas is probably correct when he says that this effort took last week's vote "out of the laugh-meter category" and created the conditions for a subsequent victory.

In maneuvering before the vote, Reagan pledged that the U.S. government would "discipline" contras involved in drug smuggling or human rights abuses, promises likely to be incorporated into the legislation before the second vote.

These promises are also tinged with irony. Despite Reagan's emotional warnings about the drug peril and his efforts to portray the Sandinistas as drug smugglers, his proposed budget reduces funds for apprehending smugglers along U.S. borders. As far as contra human-rights abuses are concerned, the administration has usually pretended that they are largely the creation of a Sandinista disinformation campaign. Recently, the president's national security affairs adviser, John M. Poindexter, was asked about these well-documented abuses and said with a straight face, "I don't have any specific information on that."

If suddenly acknowledging the abuses and cracking down on them is a necessary condition of obtaining arms for the contras, Reagan will readily agree to it. He is a man of parts, who plays the role of president with skill and conviction. Above all, he is a politician who compromises at the margins to reach his central goals.

Reaganism of the Week: Asked at a picture-taking session last Monday about his reaction to Nicaraguan President Daniel Ortega's comment that Reagan had "taken leave of his senses" in asking for aid to the contras, the president replied: "It takes one to know one."