Leading West German conservative politician Franz Josef Strauss yesterday rejected President Reagan's "zero option" plan for reducing nuclear arms in Europe in a move that seriously weakens Western European support for the U.S. proposal.

Strauss' unexpected dismissal of the Reagan proposal as "unattainable and absurd" in a radio interview left his political ally, Chancellor Helmut Kohl, as the only major West German party leader who firmly backs the U.S. position.

Kohl's rivals on both right and left, as well as some officials in Italy, now are suggesting that Washington should seek a compromise with Moscow in the Geneva talks on medium-range missiles in Europe.

The administration is sticking to its position, however, and senior U.S. and European officials recently have said privately that one reason is the U.S. desire to avoid undercutting Kohl before national elections on March 6.

The United States got a boost last week from Italian Defense Minister Lelio Lagorio, who reiterated his government's support for the "zero option" in meetings with senior U.S. officials. In Rome, however, Foreign Minister Emilio Colombo hinted that Washington should soften its position, saying that the U.S. plan could be implemented in several phases.

The Italian position is important because Italy, like West Germany, is scheduled to deploy medium-range missiles under a NATO plan that takes effect at the end of this year.

The Reagan plan calls for the Soviet Union to dismantle all of its more than 600 intermediate-range missiles targeted on Western Europe in return for cancellation of the NATO plans to deploy 572 Pershing II and cruise missiles. It is called the "zero option" because it would leave no medium-range missiles deployed in Europe.

Moscow has rebuffed the plan, refusing to take apart all of its medium-range missiles while the West dismantles none. Last month Moscow offered to cut its medium-range missile force to 162, the number already deployed by Britain and France in their individual arsenals.

Now the Soviets are asking for some compromising gesture by the United States in the Geneva negotiations on the missiles, but so far Washington has insisted that Moscow improve its offers.

Strauss was quoted by The Associated Press as saying in an interview with the Deutschlandfunk radio network that the United States should seek a compromise with the Soviets.

"The Soviet Union is not considering accepting a zero solution in the western sense of the word," Strauss said. "It is out of the question that the Soviets are ready to destroy their armaments in the necessary mass.

"What is attainable . . . is a decrease on the Soviet side of their already stationed rockets . . . so that the West can hold the extent of its rearmament in corresponding narrow limits."

Strauss also rejected the "zero option" in an address at a political rally in Munich, making a similar argument and calling the proposal utopian, Reuter reported.

Strauss' break with Kohl was particularly important because it marks a split on the missiles issue within the conservative alliance that recently came to power in West Germany in coalition with the centrist Free Democrats. Strauss heads the Christian Social Union, which has an electoral pact with Kohl's Christian Democrats and effectively is the Bavarian branch of the party.

In the heat of a campaign that has been dominated by the missiles issue, the Free Democrats already have backed off from insistence on the "zero option," and the opposition Social Democrats led by Hans Jochen Vogel have sought votes by forcefully urging a compromise with Moscow.

West German political observers said that Strauss apparently was seeking to bolster his own position with the voters in recognition that popular opinion has swung against the U.S. position.

According to recent opinion polls, 65 percent of the West German electorate favors postponing deployment of Pershing II missiles--now scheduled to begin in December--regardless of whether there has been significant progress in the Geneva talks.

The NATO plan calls for West Germany to deploy the largest number of missiles--204 out of 572--and a delay there would encourage postponement by other NATO members as well.

Italian defense minister Lagorio is convinced that Italy should start installing its cruse missiles in Sicily as scheduled early next year regardless of West Germany's decision, according to a senior Italian official. Lagorio believes that Italy should delay only if agreement is reached with Moscow on Soviet arms reductions but doubts that the Soviets will negotiate seriously before the West German elections because of the hope that a leftist victory would improve Moscow's negotiating position, the official added.

Elsewhere in Europe, French President Francois Mitterrand and British Prime Minister Margaret Thatcher have given strong public support to the U.S. position. In the Netherlands, however, officials have said that deployment may be postponed unless progress is shown in the medium-range missile talks and negotiations resume on long-range, strategic nuclear forces.