Illinois Democratic leaders announced today they have united behind a plan for Adlai E. Stevenson III to leave the party and launch an independent campaign for governor, an effort that hinges on overturning a key provision of state election law.

Calling the unexpected primary election victories last week of two Lyndon LaRouche-backed candidates on the slate topped by Stevenson "a tragic mistake," the leaders declared "emphatically and unequivocally that we cannot and will not run with these bizarre extremists."

However, they ruled out resigning from the party themselves as a way to avoid running with lieutenant governor nominee Mark Fairchild and secretary of state nominee Janice Hart, the two who are affiliated with LaRouche.

Instead, the leaders said in a statement released by Stevenson's campaign, that "we will wholeheartedly support" Stevenson for governor and two new candidates for lieutenant governor and secretary of state "on a separate line on the ballot."

"Voters will be asked to punch three numbers, rather than the customary one number, to register a vote for the true Democratic ticket in the fall," the leaders said.

The joint statement is no small achievement for a state leadership group noted far more for feuds and ambition than unity. It was signed by party candidates for office in November, including U.S. Sen. Alan Dixon, state attorney general Neil F. Hartigan, state comptroller Roland W. Burris and state treasurer nominee Jerome Cosentino.

At one time or another, each has had disagreements or strains with Stevenson or with each other. In addition, the statement said Mayor Harold Washington and his arch-rival, Alderman Edward R. Vrdolyak, as well as U.S. Sen. Paul Simon, U.S. Rep. Dan Rostenkowski and "many other Democratic leaders across the state. . . support the plan . The party is united."

The effort involves overcoming a state election law that requires independent candidates to have filed no later than Dec. 5. The Democrats said they are seeking "judicial or legislative action to eliminate the unjustifiable deadline. . . . "

Stevenson campaign manager Larry Hansen tonight said the deadline is "constitutionally suspect."

In their statement, the party officials said their lawyers think that the deadline "is unconstitutional, and we are confident that by one means or both, it will be changed."

Their goal is to move the date to Aug. 4, the deadline required of statewide candidates seeking to form a new party. If Stevenson and his supporters fail to get the date changed, he has said he will attempt to form a third party, although that option is more complicated.

In such a case, Illinois law requires a full slate of no less than 10 statewide candidates, from governor to state university trustees. The Democratic leaders rejected for themselves joining any new party, saying that "the legal and logistical barriers to such a move are prohibitive."

Stevenson has vowed not to run with Fairchild and Hart. His effort to unseat Gov. James R. Thompson (R) stalled after the LaRouche candidates defeated his handpicked choices for running mates.

Hansen said the effort begun last week to find flaws in the Fairchild and Hart filing documents continues. But he said the attempt to change the filing deadline for Stevenson "is the easiest route."

Stevenson has not yet gone to court to challenge the Dec. 5 filing deadline. That move is expected to come next week. Meanwhile, Democrats in the Democratic-controlled state legislature, which reconvenes Tuesday, are preparing legislation that would change the deadline.

The leaders said their request that voters "punch three numbers" on the Nov. 4 ballot "is a small price for a message that our party is united. . . against the madness of Lyndon LaRouche and his small band of neo-Nazis."

If successful, Stevenson's independent move would confront Illinois' 6 million voters with a Democratic ballot with a vacancy for governor, even though nominees for governor and lieutenant governor are elected together with a single vote.

Voters could pull the party-line lever, casting a vote for every Democratic nominee, then vote separately for Stevenson, his new lieutenant governor candidate and his choice for secretary of state.

Ticket-splitting is becoming more common in the Prairie State with the decline of the Chicago machine of the late mayor Richard J. Daley.

As an independent, Stevenson, a former U.S. senator who narrowly lost to Thompson in 1982, need choose only nominees for lieutenant governor and secretary of state. His original running mates, state Sen. George Sangmeister and sewer commissioner Aurelia Pucinski, daughter of Chicago alderman and former congressman Roman Pucinski, are barred by state law from running again this year because they were defeated.

If forced to found a third party, Stevenson would face the tough problem of fielding rival candidates to the statewide party slate -- including all the politicians who today signed the unity statement. "That would be a little tougher," said a Stevenson adviser. "We don't think it will happen."