Leading West German conservative Franz Josef Strauss declared today that "it is unavoidable" that the West will proceed with deployment of modern nuclear missiles in Europe later this year because "not the slightest progress has been achieved so far in the Geneva arms control talks between the United States and the Soviet Union."

In an interview with three American reporters, Strauss dismissed fears that stationing the Pershing II and cruise missiles would foment political turmoil and stressed that the weapons are necessary to prevent "Soviet blackmail of Europe in the 1980s" and to strengthen security ties between the United States and its European allies.

The Bavarian premier, whose Christian Social Union holds about 11 percent in the polls as the sister party of the Christian Democratic Union, is heading into the final weeks of the campaign to become the country's vice chancellor and foreign minister if the conservatives win an absolute majority in the March 6 elections.

The way for the elections was cleared today when West Germany's constitutional court dismissed, 6 to 2, petitions from four parliamentary deputies who had challenged Chancellor Helmut Kohl's grounds for forcing the early elections.

In his talk with reporters and earlier, in a rousing three-hour speech before 8,000 fans in this border village on the Danube, the 67-year-old Strauss voiced the strongest endorsement to date among German politicians of the controversial missile deployments.

While saying that he hopes the arms talks would enable the Pershing and cruise missiles to be deployed at "the lowest achievable level," Strauss reiterated a view that he said was expressed by former secretary of state Henry A. Kissinger that "the rift between Bonn and Washington would be incurable if the missiles were not deployed."

He said his criticism of President Reagan's zero option, which offers to cancel deployment if the Soviets scrap their SS20 missiles, was based on the conviction that such a goal was "realistically unattainable, and thus rearmament was needed to cope with the Soviet mentality and practice."

He added that the deployment deadline must not be extended or else the Soviets would "hold the negotiating keys in their hands" and cause trust in Atlantic security arrangements to dissipate.

"I have the. . . feeling for the increasing alienation of the public atmosphere on both sides of the Atlantic," he said. He went on to say that he thought it would be dangerous for Western Europe if the security of Hamburg was not as important as that of Cincinnati.

Strauss contended that "counter measures must be taken" against the SS20 missiles because the Soviets cannot be expected to destroy "weapons systems in which they have invested $30 billion in rubles."

He chastized the Social Democratic leadership for "taking Germany down the road toward de facto neutralization" and added, to roaring approval by the audience, "Thank God the Americans are negotiating for us at Geneva and not Social Democratic arms expert Egon Bahr."

Citing the dangers posed by "pacifists who shirked responsibility," Strauss warned that a Social Democratic government that ruled with support from the antinuclear Greens movement would mean for West Germany "farewell to democracy and a return to primitive forms of life."

He criticized the Greens for being "dropouts from school, society and human competition" whose aim is to disrupt the parliamentary system that has ensured a long era of postwar political stability for West Germany.

"Our blindness caused World Wars I and II, and this must not be allowed to lead to a third fatal mistake in this century."

Scattered around the hall, banners and signs bore the Christian Social Union's slogan, "We Must Bring Order Back to Germany."

Later, Strauss told reporters that such phrases should not be misconstrued because, he said, "I am not an authoritarian personality but rather a personality with an authority."

Strauss' ebullient, domineering personality has prompted speculation that if the Christian Democrats win an absolute majority he would be the most powerful member of the new Cabinet. Indeed, in the past he has jokingly remarked, "I do not care who serves as chancellor under me."

On the missiles issue Strauss said today, "I say what Kohl thinks and he thinks what I say."

Nonetheless, at campaign speeches and rallies, he has treated Kohl with respect and deference, and he has spoken frequently of the need for the Christian Democrats to restore a unified governing spirit following the squabbling that eroded the 13-year ruling partnership between the Social Democrats and Free Democrats.

In his speech today, he berated the Free Democrats' claims that they provide the last bastion of free market thinking in West Germany and charged that Otto Lambsdorff, a Free Democrat who has served as economics minister under former chancellor Helmut Schmidt and Kohl, must also bear responsibility for the "lies and fairy tales" to be found in budget mismanagement in recent years.

During his rambling address, interspersed by lusty cheers as the crowd swilled huge mugs of beer, Strauss derided the Social Democratic leadership of Schmidt, former chancellor Willy Brandt and the party's current candidate for top office, Hans-Jochen Vogel.

To the delight of the crowd, he belittled Brandt as a "foreign policy dilettante" and said his previous characterizations of Vogel as a "middle-class Philistine was far too polite."