Abortion rights activists were prepared for a climactic struggle over the successor to Justice Sandra Day O'Connor, a critical swing vote on the constitutional right to abortion. But the choice of Harriet E. Miers presented them with a very different challenge - not a clear-cut opponent of abortion, but someone with very little record on the issue at all. Most of their attention was focused Monday on what public record existed on the issue: Ms. Miers's leadership, as president of the State Bar of Texas from 1992 to 1993, in an effort to roll back the American Bar Association's support for abortion rights.