WASHINGTON — Senator Susan Collins, a moderate Maine Republican whose vote could prove decisive in filling the Supreme Court’s vacant seat, said on Sunday that she would not vote for a nominee who showed “hostility” toward Roe v. Wade, the landmark 1973 decision establishing a constitutional right to abortion.

“A candidate for this important position who would overturn Roe v. Wade would not be acceptable to me, because that would indicate an activist agenda that I don’t want to see a judge have,” Ms. Collins said on ABC’s “This Week.”

In another interview on Sunday, on CNN’s “State of the Union,” the senator said such a decision “would mean to me their judicial philosophy did not include a respect for established decisions, established law.”

“And I believe that that is the very important fundamental tenet of our judicial system, which, as Chief Justice Roberts says, helps to promote stability and evenhandedness,” she added, referring to John G. Roberts Jr., the court’s chief justice.