Facing a Rift, U.S. Spy Chief to Step Down WASHINGTON â€” Dennis C. Blair, whose often tumultuous tenure as director of national intelligence was marked by frequent clashes with White House officials and other spy chiefs in Americaâ€™s still fractured intelligence apparatus, announced Thursday that he was resigning. His exit after little more than a year comes as the White House is facing thorny decisions about Iranâ€™s nuclear program, the future of Afghanistan and the spread of militancy from Pakistanâ€™s tribal areas. It also fuels new doubts about the success, and wisdom, of the major intelligence overhaul in 2004 that created the spymaster position. The White House did not announce Thursday who would succeed Mr. Blair, but a senior administration official said the likely candidate was James R. Clapper Jr., a retired Air Force lieutenant general and the Pentagonâ€™s top intelligence official. Mr. Blairâ€™s replacement will be the fourth intelligence director in five years. The departure of Mr. Blair, a retired admiral, had been rumored for months, but was made official when President Obama called him Thursday and asked him to step down. Mr. Blairâ€™s relationship with the White House was rocky since the start of the Obama administration, and he fought a rear-guard action against efforts by the Central Intelligence Agency to cut down the size and power of the national intelligence directorâ€™s staff. He is the first high-ranking member of the Obama national security team to depart. Mr. Blairâ€™s departure could strengthen the hand of the C.I.A operatives, who have bristled at directives from Mr. Blairâ€™s office. In recent months, Mr. Blair has been outspoken about reining in the C.I.A.â€™s covert activities, citing their propensity to backfire and tarnish Americaâ€™s image. The administration has largely embraced the C.I.A. operations, especially the agencyâ€™s campaign to kill militants in Pakistanâ€™s tribal areas with drone aircraft. Born out of the intelligence debacle before the Iraq war, the intelligence directorâ€™s post was intended to force greater cooperation within a hidebound intelligence bureaucracy, and to ensure that Americaâ€™s spies were better equipped to prevent a repeat of the Sept. 11, 2001, attacks. Yet most intelligence experts agree that the job has been troubled from the start, having little actual power over the operations and budget of a sprawling intelligence infrastructure that the Pentagon and C.I.A. still dominate. The vast majority of Americaâ€™s annual intelligence budget, nearly $50 billion, is spent on spy satellites and high-tech listening devices under Pentagon control. In a statement released Thursday evening, Mr. Blair praised intelligence operatives for working â€œtirelessly to provide intelligence support for two wars and to prevent an attack on our homeland.â€ In a statement Thursday, Mr. Obama praised Mr. Blair for â€œa remarkable record of service to the United States,â€ and said he had â€œserved with great integrity, intellect, and commitment to our country and the values that we hold dear.â€ Officials said that Mr. Obama called Mr. Blair on Thursday to ask for his resignation, but that the two men had several discussions in person about the subject this week. Their relationship has been characterized as professional but not close, and some administration officials said Mr. Blair often felt cut out of discussions about important security matters. Tensions among the White House, the intelligence director and Congressional oversight committees escalated after a young Nigerian man nearly detonated a bomb on a trans-Atlantic flight on Dec. 25. White House officials openly criticized Mr. Blair and his staff for a litany of missed signals that could have prevented the man, Umar Farouk Abdulmutallab, from boarding the plane. They laid particular blame on the National Counterterrorism Center, one agency that Mr. Blair supervises. A report released this week by the Senate Intelligence Committee was particularly critical of the NCTCâ€™s failures to piece together the information that could have put Mr. Abdulmutallab on a â€œno-flyâ€ list. American officials said that Mr. Blair had also angered the White House in recent months by pushing for closer intelligence ties to France, an arrangement opposed by Mr. Obama. Some intelligence experts and Republican lawmakers say they believe that the White House has tried to micromanage Americaâ€™s spy agencies, and there was a particularly tense relationship between Mr. Blair and John O. Brennan, the White House counterterrorism director. But Mr. Blair also fought battles inside the intelligence ranks. Last summer, he clashed with Leon E. Panetta, the C.I.A. director, over the appointment of the senior American spies overseas. Mr. Panetta went so far as to issue a memorandum to C.I.A. operatives telling them to disregard a directive that Mr. Blair had sent a day earlier. Mr. Blair, a Navy officer for decades, considered Mr. Panettaâ€™s move an act of insubordination, intelligence officials said. Peter Baker contributed reporting.