Leonard DeFiore, superintendent of parochial schools for the Washington Archdiocese for the last four years, has taken a month's leave of absence to serve as director of the office of Catholic affairs for the Reagan-Bush campaign.

DeFiore, the first laymman to head Catholic schools here, said he has not been particularly active in politics but that he took the assignment because he supported Republican candidate Ronald Reagan's stand on aid to Roman Catholic schools and the abortion issue.

"When I perceived the difference between Reagan and Carter in the issues of tuition tax credits and the right to life, I decided to get involved," DeFiore said yesterday. "It just seemed to me that if we were ever going to make a dent on these things, I had to make myself available."

Archbishop James A. Hickey, who heads the Washington archdiocese, could not be reached yesterday for comment, but an aide said that Hickey had approved the leave arrangement. "He [deFiore] took a leave without salary in order to feel he could work within the political realm without implicating the archdiocese," the aide, Msgr. Raymond Boland, said.

Since DeFiore is not a member of the priesthood, church strictures against clergy involvement in secular politics, which earlier this year blocked Congressman Robert Drinan (D-Mass.) from his re-election bid, do not apply.

DeFiore said that he testified last summer on the tuition tax credit issue before the Democratic National Committee "but they didn't see fit to take my advice."

The Catholic educator said he has had no long-term ties with the Republican Party, but got the job because "I was involved in working for tuition tax credits in 1978, with the 95th Congress. As a result, I just met people who were professionals."

In 1976, a well-known urban priest, Msgr. Geno Baroni, worked actively for the election of President Carter and was subsequently appointed to a top government post.