Ronald Reagan's campaign manager suggested yesterday that the former California governor would be presented this fall as a moderate, flexible administrator to counter expected attacks by President Carter.

William J. Casey, the New Yorker who has headed Reagan's campaign since John Sears was fired in February, told reporters he assumes that Carter will try to exploit Reagan's factual flubs and conservatism, but said he was confident that Reagan could "rise above this factual criticism" and run positively on his record.

"I'm a helluva lot more concerned about a fellow who makes false promises," Casey said of Carter, "than one who makes misstatements that can be readily corrected."

Casey said Reagan had hired California adman Peter Dailey as media adviser for the fall campaign, and indicated the themes likely to be used. Calling Reagan an "adaptable, flexible chief executive" and "something of an intellectual," Casey said, "He's perceived to be more conservative and more to the right than he is."

Saying Reagan would try to draw blue-collar and middle-income Democratic voters to win in the Midwest, Northeast, Texas and Florida, Casey said Reagan's record as governor of California and his positions on current issues could be presented more favorably than Reagan has been able to during the primary season.

"Whether we'll change Ronald Reagan's views to bring that out, I don't think so," Casey said.

Dailey headed the advertising team known as the November Group, which helped reelect Richard M. Nixon in 1972. He is considered a moderate Republican who is best at running a positive ad campaign in behalf of his candidates.

The Carter campaign has said it hopes to make Reagan the main issue in November. Casey's comments yesterday were the most prominent sign that Reagan intends to run as a moderate and will try to use his record positively, rather than shrink from it.

Saying he expected "a lot of quibbles" from Carter about Reagan's record, Casey said, "We think it's a helluva record."

He said that while Reagan would concentrate on his own record during the fall campaign, Carter's record "will emerge . . . through surrogates and through television."

Casey said Reagan's advisers were considering 12 to 15 people for the vice presidential slot on a Reagan ticket, adding that the only criterion articulated by the candidate was that his running mate share a "general philosophical perspective."

Asked if Reagan's running mate would have to agree with him on the need for a constitutional amendment banning abortion and opposition to the Panama Canal treaties, Casey said, "Nobody agrees with anybody on everything." But he refused to speak for Reagan on the issue.

He also indicated that he was more flexible than Reagan on the terms of a proposed tax cut. Casey said he was wedded to the Kemp-Roth bill "as a concept," but said the timing and size of a tax cut should depend on economic conditions.

"I don't think anyone can come down on Kemp-Roth in final terms," Casey said.

Reagan has been under pressure from some of his economic advisers to modify his absolute support for a 30 percent tax cut over three years that he has advocated throughout the campaign, but so far he has resisted. On Tuesday in Oregon, he was asked again about his position, and said it had not changed.