President Barack Obama said Wednesday that the U.S. ambassador to the United Nations, Susan Rice, will succeed the departing Tom Donilon as the White House national security adviser.

He also nominated former National Security Council member Samantha Power to replace Rice as U.N. envoy.

Power's nomination is subject to Senate confirmation.

Rice, Power and Donilon joined Obama for the announcement at the White House.

Obama said that Donilon, who will leave his post in July, has performed a "Herculean task" over the past four years in coordinating national security and has been "one of the most effective national security advisors our country has ever had."

Rice is "at once passionate and pragmatic," Obama said, going on to emphasize that she has been "a tireless advocate in advancing our interests" during her time as U.S. ambassador to the United Nations.

Rice's nomination for one of the key posts in the crafting of Obama's foreign policy is an open challenge to Republicans.

Several Republican lawmakers accused Rice of lying about the attack on the U.S. Consulate in Benghazi, Libya, on Sept. 11, 2012, that killed Ambassador Chris Stevens and three other Americans.

In the face of the criticism, Rice withdrew her name from consideration to replace Hillary Clinton as secretary of state, a post for which Obama ultimately selected John Kerry.

With regard to Power, an activist against genocide, Obama said she knows the strengths and weaknesses of the United Nations and "knows that American interests are advanced when we can rally the world to our side."

"And to ensure that we have the principled leadership we need at the United Nations, I would strongly urge the Senate to confirm her without delay," the president said. EFE