BONN, AUG. 6 -- Opposition leaders in both Germanys tonight rejected West German Chancellor Helmut Kohl's attempt to advance the all-German elections from December to October, but said they will agree to complete German reunification next month.

Emerging from an all-evening strategy session, leaders of the Social Democratic Party -- the major opposition in West Germany and the junior partner in East Germany's ruling coalition -- said they will deny Kohl the two-thirds majority vote in parliament that he needs to change the Bonn constitution and hold early elections.

However, Social Democratic leader Oskar Lafontaine, Kohl's leading opponent in the scheduled Dec. 2 elections, said his party will push to unify the two Germanys on Sept. 15, one day after the four World War II powers -- Britain, France, the United States and the Soviet Union -- are set to complete their talks on German unity.

Since Friday, when Kohl and East German Prime Minister Lothar de Maiziere said that the struggling East German economy required the swiftest possible creation of an all-German government, opposition parties in both countries pledged to prevent early elections.

The opposition said Kohl's attempt to move up the election date was not a plan to aid the East German economy but a tactic designed to assure his reelection by capitalizing on his current peak of popularity.

That popularity stems in part from Kohl's recent success in gaining general approval from Soviet President Mikhail Gorbachev on reunification.

At a press conference tonight, Lafontaine said that "East Germany is on the verge of collapse." He blamed Kohl for rushing the two countries' monetary unification and producing the struggling industries and mounting unemployment that prompted Kohl and de Maiziere to propose swift unification.

"The German unity process must be made sensible," said Wolfgang Thierse, a leader of the East German Social Democrats. Thierse accused Kohl and de Maiziere of acting rashly to further their own interests rather than to help anyone in East Germany.

The Social Democrats refused to consider Kohl's proposed Oct. 14 vote because they say the chancellor is trying to spring elections on the German people before they realize the depth of East German economic problems and the true cost of unification.

The Social Democrats earlier considered pushing for unification as soon as Wednesday, when the East German parliament is to hold a special session. But party leaders said tonight that other countries deserve to have reunification occur in a planned and orderly fashion.

The Soviet Union criticized Kohl and de Maiziere's proposal last week, saying that the two Germanys should not complete unification until the four powers have completed talks on ending their legal rights and restoring full sovereignty to Germany.

The Social Democrats agreed with Kohl and de Maiziere's Christian Democrats that quicker unification may help ease East Germany's economic problems by giving Western investors confidence that they can safely invest in the East.

But Kohl spokesman Dieter Vogel said that unifying the two countries before they elect a joint parliament could make investors wary of the legitimacy of East Germany's leadership.

Vogel added, however, that Kohl will accept the decision of East Germany to join the West at any time.

The East German parliament can complete unification simply by announcing that the country is acceding to West Germany.

Earlier today Kohl backed away from a statement he made Sunday night in which he seemed to acknowledge, for the first time, that reunification might require higher taxes in West Germany.

Asked in a television interview Sunday whether he would have to give up his pledge of no new taxes to pay for German unity, Kohl said he had "no fear, if it's going to cost more, to say so publicly."

But today, Vogel said that Kohl did not intend to indicate that any tax hike was being considered. He said Bonn will have to audit the East German treasury before it can clarify the economic situation there.

Kohl also promised today not to dissolve parliament, stage a no-confidence vote in his own government, then intentionally lose the vote, a tactic that would force early elections if he could not secure the two-thirds majority vote needed to amend the constitution.

In 1983, Kohl cut short the constitutionally mandated term of parliament by using the no-confidence tactic, prompting the country's highest court to warn him against using such a strategy again.

The chancellor reportedly decided not to repeat the maneuver after a conversation with West German President Richard von Weizsaecker, whose task it would be to call for the new election date.

Kohl is to meet Lafontaine and heads of other parties in Bonn Tuesday, but government spokesman Vogel said he sees no room for compromise between the positions of Kohl and Lafontaine on the election date.

Lafontaine said that Kohl has failed the German people "up and down the line" by pushing to unite the two German economies as quickly as he has.

The Social Democrats also said tonight that they will remain in the fragile five-party coalition that has kept de Maiziere at the head of East Germany's first and last elected government.

"The coalition question is not the decisive question," Thierse said.

Opposition parties have accused de Maiziere, a Christian Democrat like Kohl, of being manipulated by the chancellor.