An informal suggestion by Sen. William S. Cohen (R-Maine) on how to reduce the number of long-range nuclear weapons in U.S. and Soviet arsenals is getting "serious consideration" in the White House, according to administration officials.

Cohen's proposal is being studied as a possible formula under which actual reductions in long-range nuclear arsenals might be achieved if the two superpowers ever reach agreement at the Strategic Arms Reduction Talks that resume in Geneva on Feb. 2, the officials said.

The START talks are being conducted parallel to U.S.-Soviet negotiations in Geneva on reducing medium-range nuclear weapons in Europe.

Under Cohen's proposal, whenever either country adds a new long-range nuclear missile or bomber to its force it would be obliged to eliminate two older weapons. The idea is to allow modernization of nuclear weapons to maintain capability to deter attack "while at the same time forcing a reduction in the actual numbers of nuclear weapons," according to Cohen, who calls the proposal a "guaranteed arms build-down" rather than buildup.

Cohen, a member of the Senate Armed Services Committee, first proposed his idea in an article on the op-ed page of The Washington Post on Jan. 3. Since then, White House officials said, "a lot of attention has been paid to it."

Although it poses some problems, the officials said, the Cohen approach has "a lot in common" with the initial proposals President Reagan made at the START negotiations when they opened last summer.

Those proposals call for both sides to cut back sharply to equal levels their intercontinental-range missile forces and the number of atomic warheads carried by those missiles. But they would also allow the United States to modernize its remaining forces with the new MX in place of older land-based Minuteman missiles, and with the new Trident II in place of older submarine-based missiles.

The publicly known START proposals do not appear to contain any formula for how such reductions would be reached. Cohen's idea has appeal because it presents a possible formula while allowing the administration to go ahead with deployment of all its new long-range missiles.

Moscow has rejected the original Reagan proposals and has put forward proposals of its own at Geneva. They would achieve significant reductions, but not as deep as those favored by Reagan.

The Reagan proposals are meant to reduce the most threatening type of nuclear weapons--the big and accurate land-based missiles that pose the threat of a first strike against an opponent's forces--in the first phase of a START negotiation.

The United States, it has been learned, also has proposed an equal limit on long-range bombers if the Soviets' new Backfire bomber is counted as a long-range weapon rather than as a medium-range aircraft as Moscow has maintained in the past.

The United States proposes to discuss other weapons in the second phase of the START talks. In now intense discussion within the administration about being more precise in Geneva about what the administration is willing to talk about in the next phase, officials said, the "hot topic" is the thousands of new cruise missiles, about 1,000 with nuclear warheads, that the Navy is planning to put aboard ships and submarines in the next few years.

Because of the profusion of these new weapons, their intended use for many different roles, and the difficulty in distinguishing between nuclear and conventionally armed versions, they pose a big problem for arms controllers.