Dennis C. Blair will resign Friday as the nation's intelligence director after a tenure marred by the recent failures of U.S. spy agencies to detect terrorist plots and by political missteps that undermined his standing with the White House.

Blair, a retired U.S. Navy admiral, was pushed out 16 months after he became President Obama's surprise pick to be the nation's third director of national intelligence. His departure is likely to renew debate over whether the DNI position, a daunting job created amid sweeping intelligence reforms after the Sept. 11, 2001, attacks, is fundamentally flawed.

Obama praised Blair's integrity in a prepared statement and said that under his leadership the nation's intelligence services had "performed admirably and effectively at a time of great challenges to our security."

Blair's offer to step down came during a phone conversation with Obama on Thursday in which the president said he planned to put someone new in the director position, according to an official familiar with the exchange. Blair's exit creates a critical national security vacancy at a time when U.S. spy agencies are under pressure to step up their defenses against emerging terrorist threats.

His departure had been rumored in Washington for months, but the nature of his resignation -- without a replacement ready to be named -- suggested a lack of coordination.

The U.S. official, who spoke on the condition of anonymity to discuss internal deliberations, said Obama had first raised the possibility of replacing Blair in discussions with him earlier this week. The White House had indicated a preference that Blair stay in the job until a successor could be named. But Blair refused after learning that the president had decided to look for a new director, the official said.

Blair issued a statement saying that it was "with deep regret that I informed the President today that I will step down." He added that during the Obama administration, U.S. spy agencies had become "more integrated, agile, and representative of American values." Blair becomes the highest-ranking member of the administration to resign.

Current and former U.S. officials said the White House had discussed the position with former senator Chuck Hagel (R-Neb.), who serves as co-chair of Obama's intelligence advisory board; James R. Clapper, a retired Air Force lieutenant general serving as undersecretary of defense for intelligence; and John Hamre, a former deputy secretary of defense who leads the Defense Policy Board.

Clapper is a leading candidate to replace Blair, a senior White House official said Thursday night, speaking on the condition of anonymity because the search continues. Officials said Hagel told the White House he would not be interested in the position. The former senator was abroad and could not be reached for comment.

Blair had been charged with carrying out Obama's campaign pledge to move the country away from controversial programs -- including the CIA's use of harsh interrogation methods -- that administration officials argued had damaged the nation's standing around the world. But much like his predecessors, Blair struggled to gain traction in a position that is widely seen as lacking adequate authority to oversee an often fractious community of 16 spy services.

Blair sometimes made public remarks that revealed his frustration with the way the intelligence community functions, and that were seen as embarrassing to the administration.

In January, Blair questioned the administration's failure to use a new team of specially trained interrogators to question the suspect in the attempted bombing of a Detroit-bound airliner on Christmas Day. The unit was created for such scenarios, Blair said in Senate testimony, adding, "And duh . . . the decision was made on the scene."