By Caren Bohan

WASHINGTON (Reuters) - The exit of top aide RahmEmanuel and other staff changes give President Barack Obama achance to signal a fresh start as he tries to boost his weakpoll numbers and prepare for his 2012 re-election campaign.

White House officials said Emanuel's departure and that ofother senior aides, such as top economic adviser Larry Summers,would mean a loss of valued players.

But outside analysts said that with Obama's approvalratings mired near 45 percent, he could use the changes tosignal he is hitting the "reset" button on his presidency at atime when his Democrats fear big losses in the Nov. 2congressional elections.

Here is a list of the changes that have taken place and arein store for the Obama administration.

WHITE HOUSE CHIEF OF STAFF RAHM EMANUEL

Obama's hard-charging senior aide plans to leave theadministration to run for mayor of Chicago. Emanuel holds broadsway over policy and legislative strategy and even over whomeets with the president. Sources said he would formallyannounce his plans Friday.

Potential successors include:

* Senior adviser Pete Rouse. The longtime Obama aide isexpected to fill in for Emanuel on an interim basis but may notend up as the permanent replacement.

* Deputy national security adviser Tom Donilon. Donilon,who has served in senior roles in the State Department and isclose to Vice President Joe Biden, has a milder temperamentthan Emanuel. But Donilon is known as one of the White House'shardest working staffers and associates describe him as highlyeffective. He has developed a strong rapport with Obama.

* Ron Klain, chief of staff to Biden. Klain also served asa top aide to former Vice President Al Gore. He is said toenjoy his current job and may not want to make a change.

* White House counsel Robert Bauer. Bauer has been alongtime adviser to Obama and served as his personal attorney.

* Former Senate Democratic leader Tom Daschle. Daschle, asenior Democrats whose counsel Obama values highly, had beennominated by the president early in his term to become HealthSecretary but a flap over unpaid taxes led him to withdraw.

* Leon Panetta, currently the CIA director, was chief ofstaff to former President Bill Clinton.

LARRY SUMMERS, DIRECTOR OF THE WHITE HOUSE NATIONALECONOMIC COUNCIL

Summers has said he will leave the administration by theend of the year to return to Harvard University, where he is aprofessor. Obama is being urged by some to tap someone with abusiness background for Summers' job, saying it would help heala rift between the White House and the business community.

But many White House aides -- including Summers -- sayObama will focus on finding the best person for the job.

Possible replacements for Summers include:

* Anne Mulcahy, a former Xerox Corp. chief executive, hasbeen considered a leading candidate. But she said on CNBCThursday that she did not feel the job was a good fit for her,and that other candidates might be better.

* Laura Tyson, a former top economic adviser to formerPresident Bill Clinton, is a professor at the University ofCalifornia, Berkeley.

* Diana Farrell is a deputy to Summers at the NationalEconomic Council. As a former analyst at Goldman Sachs andformer director at the McKinsey Global Institute, she wouldbring business expertise.

* Jason Furman, also a deputy to Summers, was a topeconomic adviser to Obama during his presidential campaign.

SENIOR WHITE HOUSE ADVISER DAVID AXELROD

Axelrod has indicated he will likely leave theadministration sometime in 2011 to help lay the groundwork forObama's 2012 re-election campaign.

Many speculate that White House press secretary RobertGibbs would then move into a more behind-the-scenes roleadvising Obama on strategy and communications. Gibbs' deputy,Bill Burton, is seen as a top contender to fill Gibbs' jobbehind the podium.

GENERAL JAMES JONES, NATIONAL SECURITY ADVISER

Democratic sources say Jones is likely to leave his post bythe end of this year. He is the point person for coordinatingforeign policy advice to the president. Replacements could be:

* Donilon, who is also a leading candidate for White Housechief of staff, could be considered for a promotion to Jones'job.

* General James Cartwright, vice chairman of the JointChiefs of Staff, is referred to by Obama as his "favoritegeneral," according to a new book by journalist Bob Woodward.

* Deputy Secretary of State James Steinberg. Steinberg wasdeputy national security adviser in the Clinton administrationand advised Obama during his 2008 campaign.

DEFENSE SECRETARY ROBERT GATES

Gates, a holdover from the Bush administration, hassignaled his intention to resign sometime in 2011.

Contenders to replace him include:

* Michele Flournoy, undersecretary of Defense for policy,who would become the first female Defense chief.

* Democratic Senator Jack Reed, an Army veteran. Reed issomeone whose counsel Obama values, Democrats say.

* Chuck Hagel, a former Republican senator and a VietnamWar veteran.

ADMIRAL MIKE MULLEN, CHAIRMAN OF THE JOINT CHIEFS OF STAFF

Mullen, the top U.S. military officer, is expected toretire once he completes his second term on Oct. 1 next year.

* General David Petraeus, currently commanding the war inAfghanistan, would seem to be a natural fit for the job but itis unclear whether Obama would want to pull him from the fieldafter just over a year.

* General James Cartwright, vice chairman of the JointChiefs of Staff, is considered a strong contender for Mullen'sjob.

TOP ADVISERS WHO ALREADY HAVE LEFT THE ADMINISTRATION

* Christina Romer departed as chairwoman of the Council ofEconomic Advisers at the beginning of September. She wasreplaced by Austan Goolsbee, who had been a member of the CEA.

* Peter Orszag, manager of the White House Office ofManagement and Budget, left in July. Obama has nominated JacobLew, deputy Secretary of State for management and resources andchief operating officer of the State Department, to replacehim; Lew has not yet been confirmed by the Senate. (Additional reporting by Phil Stewart and Patricia Zengerle;Editing by Xavier Briand)