Morris P. Fiorina, a specialist in American voter behavior at the California Institute of Technology, yesterday accepted an offer of tenure from Harvard's Government Department and will join the Faculty in September 1983.

Fiorina said he notified Henry Rosovsky, dean of the Faculty, of his decision in a telephone conversation yesterday.

Fiorina's appointment, which the Corporation still must formally approve, fills a position in American government left vacant by Martin Shapiro, who left the Faculty in 1974.

John D. Montgomery, chairman of the Government Department, would not say whether the department had made previous offers to fill the opening. "We've had a lot of debates and discussions over how to fill the slot over the last eight or nine years," he said.

"Fiorina's not a second choice," he added.

Montgomery, who declined to confirm that Fiorina had accepted Harvard's offer said the department specifically sought a scholar with quantitative methods of research to fill Shapiro's position. But the search did not reflect a broad effort on the department's part to find more quantitative researchers, he said.

"Some departments have gone all out to find quantitativists with an almost religious zeal, and many of them have taken the view that we've been laggards," Montgomery said.

'We've move cautiously but more effectively," he added.

Fiorina will spend next year on a Guggenheim Fellowship at Stanford's Center for Advanced Studies in Behavioral Sciences, conducting research on how governmental regulations originate. The 36-year-old political scientist is the author of three books about the motives of American voters.

Asked to describe his political beliefs. Fiorina said, "I'm philosophically close to being a libertarian, but I recognize many of those views are impractical."

"I have a skepticism for the more or less automatic use of government," he said, "but in a practical world, there are problems the government has to address, such as environment, health and safety."