WASHINGTON – President Bush announced his selection Tuesday of Condoleezza Rice to replace Colin Powell as secretary of State, tapping one of his closest and most trusted aides at what he termed “a critical time” for America.

If she is confirmed by the Senate, the move would put Rice fourth in the line of succession to the presidency.

Rice, 50, the president’s national security adviser, is expected to win Senate confirmation easily. She is likely to be grilled, however, about her failure to respond more aggressively to the threat of terrorism before Sept. 11, 2001, and about her role in the buildup to the Iraq invasion.

As expected, Bush named Rice’s deputy, Stephen Hadley, to replace her as the top foreign policy adviser on the White House staff.

In making the announcement, with Rice at his side, Bush called the Alabama native “the right person” to confront a daunting array of global problems, including the fight against terrorism, the Arab-Israeli conflict and nuclear arms proliferation.

“We’re a nation at war,” said Bush, who cast his Iraq policy as a response to “a great calling of history” to reform the wider Middle East. He added that, in Rice, “the world will see the strength, the grace and the decency of our country.”

Rice would be the only national security adviser, other than Henry Kissinger, to move directly into the top job at the State Department. Powell also served briefly as White House security adviser, at the end of Ronald Reagan’s second term.

Rice would become the second African-American, after Powell, to head the State Department and the second woman, after Madeleine Albright.