WASHINGTON (CNN) -- Sen. John McCain, R-Arizona, told the Associated Press Monday that he thinks it's unlikely a pro-choice Republican can win the party's presidential nomination.

McCain's comments came hours before a Politico.com article was published reporting that one of McCain's chief rivals for the GOP nomination, former New York City Mayor Rudy Giuliani, had donated to the pro-choice group "Planned Parenthood" multiple times in the 1990's.

"I think it's one of the fundamental principles of a conservative to have respect and commitment to the dignity of human life, both the born and unborn, "McCain said. "It makes it tough because the Republican Party is basically composed to a significant degree by people who are pro-life, just as the Democratic Party has pro-choice candidates."

Giuliani has maintained that he personally is against abortion, but respects the right of a woman to make that decision.