"I do not support Roe versus Wade. It should be overturned," the Arizona senator told about 800 people in South Carolina, one of the early voting states.
McCain also vowed that if elected, he would appoint judges who "strictly interpret the Constitution of the United States and do not legislate from the bench."
The landmark 1973 decision in Roe v. Wade effectively legalized abortion in the U.S. The Supreme Court has narrowly upheld the decision, with the presence of an increasing number of more conservative justices on the court raising the possibility that abortion rights would be limited.
Social conservatives are a critical voting bloc in the GOP presidential primaries.
McCain's campaign also announced early Sunday that he had been endorsed by former Oklahoma Gov. Frank Keating, who had been considering his own bid for the White House, and former Texas Sen. Phil Gramm, who failed in his bid for the Republican nomination in 1996.
Keating told the crowd that McCain is the "only candidate who is a true-blue, Ronald Reagan conservative."
McCain later attended an evening rally promoting an abstinence program. He told the crowd of more than 1,000 teens and parents that young people have pressures far different from the ones he faced while growing up. "Sometimes I've made the wrong choice," McCain said.
He also talked about his experience as a prisoner of war during Vietnam, and described some of the torture he suffered. His captors "wanted to make us do things that we otherwise wouldn't do," including confessing to war crimes, McCain said.
He and fellow prisoners were beat up for practicing their religion, but they continued to do it. "Sometimes it is very difficult to do the right thing," he said.
McCain has strong name recognition and the largest network of supporters in South Carolina. That backing comes in part from his staunch support for the Iraq war, something on which he focused a day earlier in Iowa. But it's the same state that dealt a crushing blow to his presidential aspirations in 2000.
McCain is trying to build support among conservatives after a recent rebuke from Christian leader James Dobson, who said he wouldn't back McCain's presidential bid. Conservatives question McCain's opposition to a constitutional amendment to ban gay marriage. He opposes same-sex marriage, but says it should be regulated by the states.