On the eve of two primaries that could clinch the Republican presidential nomination for him, Ronald Reagan spent the day ignoring George Bush and attacking the Carter administration.

In Detroit, Reagan assailed the administration for "continuing devotion to job-killing regulation" in the auto industry, and said that the federal government should eliminate "arbitrary year-by-year mileage standards" for American cars. Reagan said that it would be sufficient to keep the federal 1985 goal of 27.5 miles per gallon for passenger vehicles and allow normal requirements of competition to promote fuel efficiency within this standard.

In Portland, Reagan turned to Carter economic policies, which he said were producing a "credit crunch" for small businesses.

"Our goal should be to help the small businessman survive, not sink him," Reagan said. "Forcing small business to close up shop eliminates our greatest opportunity for widespread economic growth and more jobs."

Reagan was his customary cautious self, however, when reporters in both cities asked him whether he thought he would go over the top in Tuesday's primaries in Michigan and Oregon. Most of the time he repeated, with a smile, an old line that he was "cautiously optimistic."

Michigan will select 82 delegates to the Republican National Convention and Oregon 29 in proportional primaries. The latest United Press International count shows Reagan with 939 delegates, 59 short of the 998 needed for nomination.

Reagan, who lost both of these primaries to Gerald Ford in 1976, is a slight favorite over Bush in both states although there are some who think that Bush could upset him in Michigan on the basis of heavy media spending there.

However, Reagan conceivably could clinch the nomination even with a narrow loss in Michigan.

Reagan's refusal to say that he has the nomination wrapped up is based on his observation that no two delegate counts are precisely alike. But when it was pointed out to him that he is likely to have more than the required 998 figure in many of the counts, Reagan replied: "If it should turn out that way tomorrow night and you all say so, I'll sleep better."

A better reflection of the Reagan camp's true opinion of the political situation is shown in the schedule of the former California governor, who for the first time in many primaries is scheduled to spend Tuesday night at his remote ranch near Santa Barbara.

Reagan continued to refuse to say that Bush should get out of the race.

"I'm not going to ask him," Reagan said. "You will have to check with him on that."

In Detroit, Reagan issued a detailed statement criticizing various federal regulations he said were crippling the American automobile industry.

He said that the requirement for "elaborate new passenger restraints such as automatic seat belts and airbags" discriminates against U.S. automakers because its first application is to large cars in 1982. The small foreign imports wouldn't be affected until 1984, he said.

At a news conference in Dearborn, Reagan also expressed skepticism about the value of rail rapid transit systems.

"I am not a great fan of that," Reagan said. "We've had experience in California with a gigantic white elephant that cost several times more than it was supposed to cost."

Reagan was referring to the Bay Area Rapid Transit system, which links San Francisco, Oakland and several suburban communities.

At Eugene, facing a rare hostile crowd, Reagan was booed and jeered by students at the University of Oregon. He vigorously defended himself against accusations that he was antienvironmentalist but appeared to become angry when one student questioner walked away from him while Reagan was defending the deposed shah of Iran.

"You don't want to hear the truth," Reagan said. "That's why you're stupid."

Minutes later Reagan interrupted his own answer to another question to apologize for this response.

"I said stupid," Reagan said. "I shouldn't have said that. It was very impolite. He was just rude."