Sen. John McCain is blocking the confirmation of President Barack Obama's nominee to be America's second-highest ranked diplomat.

McCain says Tony Blinken, Obama's deputy national security adviser, is "totally unqualified" to serve as Secretary of State John Kerry's understudy.

McCain also criticizing Blinken Thursday for comments he made more than two years ago touting Iraq's security, prosperity and democracy.

McCain has been among Obama's fiercest foreign policy critics. He will assume the chairmanship of the Senate Armed Services Committee next month when Republicans take control.

Senators can put a hold on a nominee, blocking final action.