Conservatives contend that federal judges have upset the constitutional balance of power among the courts, the Congress and the presidency by making far-reaching decisions, such as one in 2005 that let cities seize people's homes to make way for shopping malls.

"My nominees will understand that there are clear limits to the scope of judicial power, and clear limits to the scope of federal power," McCain said Tuesday in a speech at Wake Forest University.

McCain, the eventual GOP nominee, promised to appoint judges in the mold of Roberts and Supreme Court Justice Samuel Alito, saying they would interpret the law strictly to curb the scope of their rulings. While McCain didn't mention abortion, the far right understands that such nominees would be likely to limit or perhaps overturn the Roe v. Wade decision that legalized abortion.

Obama, on the other hand, voted against Roberts and Alito. So did Obama's rival, Hillary Rodham Clinton, but McCain focused on Obama.