Sen. John McCain blocked a vote Thursday for a nominee to the State Department's No. 2 post, challenging White House aide Tony Blinken's qualifications and past approach to U.S.-Iraq relations.
The Arizona Republican told reporters that he considered Blinken "totally unqualified."
McCain has sharply criticized Blinken's claims earlier in the Obama administration that Iraq had become far more prosperous and democratic, permitting the withdrawal of U.S. forces in 2011.
McCain, one of the most outspoken Republican critics of the administration's foreign policy, disclosed his move shortly before the Senate Foreign Relations Committee was to vote on the nomination.
As a senator, and a member of the committee, he can block nominations indefinitely by putting a "hold" on them.
Blinken, 52, is a longtime foreign policy specialist who has worked in the Clinton and Obama White Houses and on the Senate Foreign Relations Committee staff.
He most recently served as deputy national security adviser to Obama. Before that, he was Vice President Joe Biden's top foreign policy aide. He also worked for Secretary of State John F. Kerry when Kerry was a Democratic senator from Massachusetts.
Blinken ascended steadily in the Obama administration. He was to replace William J. Burns, a respected foreign service veteran who worked at the top of Democratic and Republican administrations.
Blinken was given a visible role in Obama's first administration by calling attention to improvements in Iraq. The White House viewed stabilization of the country, and the withdrawal of U.S. forces at the end of 2011, as as one of Obama's foremost foreign policy accomplishments.
But critics, including McCain, began denouncing those claims by last year, when internal political strains in Baghdad became more corrosive. The complaints only increased when Islamic State militants swept out of Syria last spring and captured broad areas of Iraqi territory.
McCain this week stepped up his criticism of the administration foreign policy.
He condemned the administration for nominating Colleen Bradley Bell, producer for a TV soap opera, "The Bold and the Beautiful," as U.S. ambassador to Hungary.
Referring to Hungary's authoritarian leadership, McCain said, "Here we are, a nation that's on the verge of ceding its sovereignty to a neo-fascist dictator and we're going to send the producer of 'The Bold and the Beautiful'?"
The Hungarian government reacted by summoning the current U.S. ambassador to complain about McCain's comments.