President Reagan last week virtually swept off the diplomatic checkerboard 18 months of maneuvering over a summit meeting with Soviet President Leonid Brezhnev.

At his press conference Wednesday, the president went further toward questioning the likelihood of a summit meeting with Brezhnev than he has done since he took office, saying, "I don't know whether it's going to be this year or next or at all." That downgrading of a summit conference barely registered on the American scene, but it has significant implications in politics as well as in diplomacy.

All American presidents have been eager to deal with their superpower adversaries at first hand, and Reagan presumably shares that curiosity. Their advisers traditionally have been fearful that summit meetings will produce either unwarranted euphoria, or a stumble into crisis.

Only a few months ago, administration political advisers were signaling that those risks were worth taking for the domestic political benefits that could flow from a Reagan-Brezhnev summit in advance of the Nov. 2 elections. The president's latest remarks indicate the opposite approach, that little political weight is currently given to the need to register a reduction of American-Soviet tension.

The Reagan comments also show that his administration no longer feels obliged to hold out the prospect for an early summit with Brezhnev for the sake of Atlantic Alliance unity.

Before Reagan's trip to Europe in June, he and his advisers strongly encouraged expectations about a summit. On May 9 in a major commitment to "dialogue" with the Soviet Union, the president said that if it proved impossible "to meet with President Brezhnev in New York next month," as he had proposed, "I would hope we could arrange a future meeting where positive results can be anticipated."

The prospect of a summit conference, coupled with decisions to open two nuclear arms control negotiations in Geneva, did help the administration to allay widespread apprehensions in Europe about its entire East-West policy. But now the administration's priorities have changed. The loser in the summit maneuvering is the Soviet Union, although history conceivably may register a lost opportunity for both nations in bridging the great gulf between them.

For the Soviet Union, a formal summit meeting is a means of restoring some of the international stature it lost by its invasion of Afghanistan in 1979, and by the imposition of martial law in Poland last December. It is precisely for those reasons that the Reagan administration has looked on a summit meeting as a benefit to the Soviet Union which it could grant or withhold.

The Soviet Union, therefore, never seriously considered sending its ailing 75-year-old leader to New York in June, to the kind of summit meeting that Reagan wanted, informal talks on the edges of the United Nations General Assembly's special session on disarmament. Brezhnev wanted a full-dress summit meeting with Reagan, and proposed holding it in October, suggesting Switzerland or Finland as neutral sites.

In May and June, while the Reagan administration was repairing its own strained relations with its Western European allies, it indicated it was receptive to Brezhnev's alternative. But when the president returned from Europe, his administration shifted to the opposite tack. That was publicly reflected in a Reagan speech to the United Nations on June 17, in which he assailed the Soviet Union for a record of "imperialist adventures" and a "scourge of tyranny."

At the same time, senior administration officials in private stressed the necessity of producing tangible progress in American-Soviet relations as a prerequisite for any summit conference, notably, movement toward acceptance of American terms in the new strategic arms reduction talks (START) in Geneva. The chances of producing any such results by October, or any time this year, are virtually zero. Those negotiations are still in the preliminary stage, with no sign of movement toward the American demands.

As a result, although neither nation admitted it at the time, the ascent toward a summit was stalled completely when Soviet Foreign Minister Andrei A. Gromyko and Alexander M. Haig Jr., then secretary of state, met in New York on June 18, the day after the Reagan speech to the U.N.

Reagan has now publicly reconfirmed that the route to a summit is blocked, by reverting to the administration's earlier stiff terms for a meeting with Brezhnev. In the president's newly negative characterization, a summit conference not only is no "cure for everything that's wrong in the world, but it has to be carefully planned, an agenda has to be set and that begins with foreign ministers meeting."

This literally carries the process back to square one.

A new secretary of state, George P. Shultz, has to find his way through the maze of complexity that envelops nuclear weaponry and all other issues on the American-Soviet plate, before he can even begin to relaunch the elusive search for a summit.