Geraldine A. Ferraro, making her last campaign swing through the industrial Midwest, today warned that "you can't trust Ronald Reagan, the surprise president."

The Democratic vice-presidential nominee again decried a contingency tax plan reportedly approved by Treasury Secretary Donald T. Regan that would tax workmen's compensation benefits and repeal federal income tax deductions for state and local taxes.

Ferraro told a rally of union supporters that the tax plan is "hitting below the belt" and that "under Ronald Reagan's plan, you'd be taxed twice on what you earned."

"He said he'd be fair. We said we doubted it, and now we know for sure he won't be," Ferraro told the generally lukewarm crowd

Ferraro also said the "CIA taught guerillas in Nicaragua the ABCs of terror, kidnaping and assassination." She said "no one has lost his job yet" despite Reagan's promise to punish those responsible.

Later, during a rally in Green Bay, Wis., Ferraro said, "The progressive Republican Party that was founded in this state bears no resemblence to the radical right-wing outfit that calls itself Republican.

"When I started my campaign," she added, "Walter Mondale said, 'Gerry, just be yourself.' He's never asked me to change my style and, thank goodness, he's never asked me to be like George Bush."

Ferraro is spending the final 72 hours of the campaign trekking through Ohio, Wisconsin, Illinois, Michigan, Rhode Island, Connecticut, New Jersey and Pennsylvania, before making a final appearance Monday night at her alma mater, Marymount College in New York.

Ferraro and her staff appear to remain in high spirits despite adverse polling data. Press secretary Francis O'Brien has been telling reporters that "after three drinks, I'm seeing some movement in the polls."

On a flight from San Francisco to Dayton Thursday night, several aides joined the press in an impromptu conga line down the aisle of the airplane to the cockpit, past the chuckling candidate.

There was, however, a near mutiny when the candidate, staff and Secret Service agents were served dinner on the plane with glass wine goblets and linen, while the press, which pays the campaign 1 1/2 times first-class air fare, got plastic trays and cups.

Amid a lusty chorus of boos and charges of "closet Republicanism," staff aide Peter Scher quelled the rebellion by distributing tiny table cloths and real glasses to the reporters and cameramen.