President Obama announced Friday that Rahm Emanuel, his chief of staff and a fearsome White House operative, is resigning his post and would be replaced with another senior advisor. Emanuel, who is planning to run for mayor of Chicago, departs 20 months into Obama's presidency and leaves as one part of a staff shuffle that will bring a significant turnover at the top levels of the White House policy and economic team. Senior presidential advisor David Axelrod is planning to leave the White House next year to begin preparations for Obama's 2012 reelection drive, and economic advisor Lawrence Summers is quitting the White House to return to Harvard University. Obama named senior advisor Pete Rouse to serve as Emanuel's replacement, at least for now. Emanuel's departure had been expected since Mayor Richard Daley announced in September that he would not run for reelection. Obama lavished praise on Emanuel for his work at the White House. "He just brings an unmatched level of energy and commitment to every single thing he does," Obama said after embracing Emanuel before a cheering White House assembly. Possible candidates for the permanent job include Thomas E. Donilon, a deputy national security advisor; Robert Bauer, White House counsel; Tom Daschle, a former Senate Democratic leader; and John Podesta, a former chief of staff to President Clinton.