WEST BERLIN, MAY 28 -- Oskar Lafontaine, the leading challenger to West German Chancellor Helmut Kohl, has sparked a political storm that could result in his removal as candidate even before he recovers from an attempt on his life last month.

The Social Democratic Party (SPD), which chose Lafontaine for his brash style and populist touch, fell into public disarray today over the candidate's insistence that the party vote against the treaty under which the two Germanys are to unite economically July 1.

Although Lafontaine's rhetoric against quick unification has sharpened, his latest position would put the SPD on record against the treaty without endangering ratification.

Lafontaine, 46, who is premier of the Saarland, one of West Germany's smallest states, told interviewers this weekend that the treaty will cost West Germans too much and will not provide enough job protection for East German workers.

Prominent figures in the party worry whether Germans on both sides will consider the Social Democrats the party that opposed unification. East Germany's Social Democrats today implored their Bonn colleagues to support the treaty and East German Foreign Minister Markus Meckel, chairman of the East Social Democrats, called Lafontaine's strategy "politically irresponsible."

Kohl, in Berlin tonight to meet with East German Prime Minister Lothar de Maiziere, said the planned introduction of the West German mark in East Germany must proceed on schedule.

"The expectations of the people are enormous," Kohl said, adding that it would be "catastrophic" if the July 1 merger of the two economies were delayed. "We would have hundreds of thousands of refugees" from the East, Kohl said.

Opponents in Bonn of Lafontaine's tactics leaked critical stories to nearly every West German newspaper. Reports speculated that Lafontaine might be dumped in favor of West Berlin Mayor Walter Momper, who supports the economic unity treaty.

But Lafontaine, who has not taken part in any public meetings since he was stabbed in the neck a month ago, stood firm, and the party spokesman tonight insisted there was no split in the SPD. Momper is quoted in Tuesday's edition of the Bild newspaper as saying the party's "differences appear much greater to the public than they are."

Lafontaine can point to new opinion polls in both Germanys to show that his anti-treaty arguments strike a favorable chord among the voters. The newsweekly Der Spiegel reported today that 71 percent of West Germans are not prepared tosacrifice for German unity.

With fear of the impact of monetary union rising, a majority of those polled in both countries this week said they would choose Lafontaine over Kohl for chancellor. The West Germany-only election is scheduled for Dec. 2.

Other SPD leaders fear, however, that Kohl's Christian Democrats could portray the challengers as unpatriotic if they reject the treaty.

Under Lafontaine's latest proposal, SPD members would vote against economic unification in the lower legislative house, where the party is in the minority, and vote for it in the upper house, which the SPD controls, thus leaving passage unimpeded.

But several SPD parliament members openly rejected their leader's proposal. Party vice chairman Herta Daeubler-Gmelin said it was "not well thought out."

Last week, Lafontaine had threatened to quit as candidate unless the party leaders agreed to oppose the proposed treaty. With considerable internal dissent, the SPD followed its candidate and demanded stronger protection for East German workers and an agreement to impose Western environmental standards in the polluted East.

Tonight, Kohl and de Maiziere discussed some of the changes the SPD recommended, even though Kohl last week said the treaty could not be altered.