In her second visit to the Show Me State in eight days, Geraldine A. Ferraro invoked the ghost of Harry S Truman today by predicting that the underdog Democrats will score an upset on Nov. 6 like the one engineered by the onetime Missouri haberdasher in 1948.

A placard exhorted Ferraro to "Give 'em hell, Gerry" as she arrived at Truman High School for a question-and-answer session with about 1,000 students, one of whom asked for guidance in deciding whether to register as a Republican or a Democrat.

"How do you feel about the nuclear arms race?" she asked the young man.

"I don't know. I don't think much about it," he responded.

"You don't think much about it? Why not?" Ferraro persisted.

"Too busy with homework."

"I would suggest, then, that you register as a Republican," she said.

Ferraro caused some confusion with her answer to a question from a sophomore about the circumstances under which she and running mate Walter F. Mondale could foresee a first-strike nuclear attack by the United States.

In general, both Republicans and Democrats have ruled out a "first strike" but not "first use."

"First strike" usually is defined as a preemptive, surprise attack intended to destroy the enemy, whereas "first use" often is seen as the initial detonation of a tactical nuclear warhead during a conventional battle.

In replying to the student, Ferraro said: "I think that's the type of question that one does not sit down and lay out circumstances for a first strike . . . . I'm also sure that Mr. Reagan and Mr. Bush wouldn't discuss conditions that we would take as being the level at which we would move for a first strike . . . .

"I can't think that we would think of any circumstances under which we would attack . . . . 'First strike' is not in any of our vocabulary, I don't think . . . . We don't want to ever first strike."

During a subsequent news conference in Kansas City, Ferraro said: "A first strike is when you decide you're going to take all your weapons and throw them at the Soviet Union. Now I'm not going to describe the conditions under which that could happen, and I don't see us doing it."

She also said she would not discuss conditions under which a first use would be appropriate "because to lay out conditions under which you would do that is, I think, bad policy."

Barry Carter, a foreign affairs adviser traveling with the vice-presidential candidate, later told reporters: "The clear intent -- and I talked to her afterward -- is that she rules out first strike. She thought she had. She intended to."

Asked at the high school how she reconciles her Roman Catholicism with her pro-choice position on abortion, Ferraro replied: "My policies are not the policies of the Catholic church, quite frankly. . . . If I were ever to be raped and became pregnant as a result of that rape, I don't think I'd be so self-righteous. Or I don't know if I would be . . . . I don't think I should impose my religious views on people who don't share them."

Ferraro closed her appearance at the school by recalling the erroneous Chicago newspaper headline after Election Day in 1948 that declared Thomas E. Dewey the winner.

"But the next day Harry Truman got up and he was the president of the United States. I think that's what's going to happen in our campaign," she said.