"As someone who grew up in an era of comparative bipartisanship and as a sitting U.S. national security official who has served in two U.S. administrations, I am saddened that we have reached this point, even before you have decided whom to nominate," she wrote. "We can not afford such an irresponsible distraction from the most pressing issues facing the American people."

Rice served in President Bill Clinton's administration on the National Security Council, eventually leading African affairs for the council and serving as a special assistant to the president.

She was confirmed by the Senate as the top diplomat to the U.N. in January 2009.

If Rice had been nominated, she would have faced criticism from religious leaders about her role in the Clinton administration's handling of the Rwandan genocide in the 1990s.

U.S. Rep. Mike Rogers, chairman of the Permanent Select Committee on Intelligence, said on CNN that Rice's decision was "probably for the best."

"I think Ambassador Rice was facing an uphill battle in the senate for any confirmation she may have received," he said.