Roberts, 50, was nominated by President Bush to succeed Chief Justice William Rehnquist, who died of cancer Sept. 3 at the age of 80.
Much of the initial questioning by Sen. Arlen Specter, R-Pa., who supports abortion rights, involved the principle of "stare decisis," or respect for precedent, as applied to the Roe vs. Wade case, the 1973 Supreme Court decision that legalized abortion.
Roberts said a 1992 Supreme Court decision, Planned Parenthood vs. Casey, was "itself a precedent that would be entitled to respect," the Washington Post reported.
Monday, the conservative Roman Catholic Roberts used baseball metaphors, at one point comparing judges to umpires, who are there to uphold the rules, not make them.
Roberts is widely expected to win full Senate approval in time to join the court before its next term begins Oct. 3.