President Reagan supposedly has won a major victory on the MX missile, and he appears to be a big loser on his proposal to aid opponents of the Sandinista government in Nicaragua. But the reality in both cases may be the opposite of the surface appearance.

Reagan appears near the end of his rope in his long attempt to pry aid for the Nicaraguan rebels out of a reluctant Congress. Needing to cover his request for $14 million in military aid for the insurgents, he came up with a fig leaf of beguiling transparency. In essence, the president slapped a "humanitarian" label on an aid request that would allow the rebels to use privately raised funds to buy arms.

It is easy to understand why a member of Congress might support aid to the rebels, given the hemispheric importance of Nicaragua. But it is difficult to comprehend how anyone opposed to this aid, on strategic or moral grounds, could rationalize voting for it just because Reagan has renamed his request "the Central American peace plan." Nevertheless, appearances deceive and Reagan continues to amaze. Admirers and critics alike who studied Reagan's performance as governor of California expected him, once he became president, to cut the federal budget and raise taxes, exactly the opposite of what he has done.

Some would-be prophets, including this reporter, also expected him to slow down in a second term. Some prophets, some slowdown. At 74, Reagan is acting like a man in a hurry and a president in charge. He is seeing more members of Congress and more reporters than ever before. He is selling his programs even when he appears not to have the votes.

But Reagan has lost on the MX, even though his plea to give him a "bargaining chip" in the Geneva arms control negotiations postponed the day of reckoning. After inheriting a program that would have deployed 200 of the massive missiles in multiple shelters, Reagan scrapped the plan because it was favored by former president Jimmy Carter and opposed by Reagan's western supporters. Now, the president has won narrow approval for deployment of 21 missiles -- a fraction of what his predecessor had in mind. Reagan could have had these, without wasting political capital, during the honeymoon of his first term.

Instead, the MX has become entangled in the budget process, and even its supporters advocate "stretching out" deployment. The official Reagan goal of deploying 100 MX missiles is beyond reach, and the fight in Congress is between fewer than 50 missiles or none. Reagan has won nearly every battle on the MX. He is about to lose the war.

Curiously, the situation may be reversed on Central American policy, where Reagan has lost a multitude of battles while gradually gaining political command. Last week, for instance, on the same day the president unveiled his Nicaragua "peace plan," Reagan critic Sol Linowitz, the former ambassador who negotiated the Panama Canal treaty, was praising the impact of Reagan's policies in El Salvador.

The Nicaragua proposal, preserving the hope of a hollow negotiation and rechristening the military-aid proposal, has breathed new life into the once-moribund request for covert aid to the insurgents. Reagan is keeping a distance from the rebels, whom he used to call "freedom fighters" and "our brothers" and now refers to as "contras" or the "democratic resistance movement." But he has demonstrated that he is not about to abandon his basic goal of "stopping communism" in the hemisphere.

A Democratic congressional aide summed up the situation by saying the Democrats could "win the vote and lose the issue." He said Reagan could prevail by establishing himself as the defender of the hemisphere and convincing Americans that the Russians are coming to Nicaragua.

Reagan may be skillful enough to pull this off, especially if he convincingly rules out use of U.S. troops. Spurred by deep convictions about what he sees as a Soviet menace in Nicaraguan form, the president appears to be winning the public relations battle despite poll results that question his policies.

Last week, the president brushed aside these surveys, saying they reflected the success of a Soviet-Cuban "disinformation network that is beyond anything we can match." It was an odd claim by the Great Communicator. Judged by his peace plan, he seems more than a match for any disinformation system around.

Reaganism of the Week: In the Post interview last Monday, Reagan explained how being governor of California had prepared him for the presidency: "For eight years somebody handed me a piece of paper every night that told me what I was going to be doing the next day."