* Bill Clinton's philanthropic summit in NY Sept. 23-25
* Obama, Romney to address separate sessions on final day
* Nigerian, Mexican, Rwandan presidents also due to attend
By Michelle Nichols
NEW YORK, Sept 10 (Reuters) - U.S. President Barack Obama and Republican rival Mitt Romney are expected to set aside their political differences later this month to speak at Bill Clinton's eighth annual philanthropic summit.
Obama and Romney were invited earlier this summer to attend the Clinton Global Initiative (CGI), a summit official said, well before Clinton's Democratic National Convention speech last week where he gave a rousing endorsement of Obama and a detailed attack on the Republican presidential candidate.
Obama and Romney will address separate sessions on Sept. 25, the final day of the three-day summit in New York City that brings together heads of state, business leaders, humanitarians and celebrities to make commitments to tackle the world's woes.
It was not known what topics they planned to speak about.
"I'm grateful that President Obama and Governor Romney are taking time to join leaders from all parts of society who choose to address our greatest global challenges through the Clinton Global Initiative," Clinton said in a statement on Monday.
"CGI is built on the spirit of non-partisan, cross-sector collaborations that drive action and I'm proud that, since we began in 2005, CGI members have made more than 2,100 commitments that are already improving the lives of 400 million people all over the world," he said.
Clinton, who this week is due to campaign for Obama in the critical swing states of Florida and Ohio, gave point-by-point criticism of Romney and his vice presidential running mate, congressman Paul Ryan, during a prime-time address to the Democratic convention in Charlotte, North Carolina.
Relishing being back in the political spotlight, Clinton said Obama should not be blamed for the poor economy he inherited in 2009 and has set the foundations for strong growth - if voters will give him more time and re-elect him on Nov. 6.
But campaigning is expected to be set aside for the Clinton Global Initiative. A summit official said the philanthropic summit was traditionally a non-partisan event.
"The president of the United States, G20 heads of state and U.S. presidential nominees of both parties have always been invited," the official said.
"Senator Obama and Senator McCain both addressed the CGI annual meeting in September of 2008," he said, referring to 2008 Republican presidential nominee John McCain.
"The invitations to President Obama and Governor Romney for the 2012 annual meeting were extended earlier this summer, prior to the conventions," the official said.
Clinton's wife, Secretary of State Hillary Clinton - who lost the Democratic presidential nomination to Obama in 2008 - is also slated to appear at the Clinton Global Initiative, along with her Republican predecessor at the State Department, Condoleezza Rice.
The world's richest man, Mexican tycoon Carlos Slim, U.S. Treasury Secretary Tim Geithner, Rwandan President Paul Kagame, Nigerian President Goodluck Jonathan, Mexican President Felipe Calderon, Nobel Peace Prize winner Leymah Gbowee, Spain's King Juan Carlos and Jordan's Queen Rania are also due to attend.
The idea for the summit came from Clinton's frustration while president from 1993 to 2001 at attending conferences that prompted no action. So far the Clinton Global Initiative says it has produced more than 2,100 pledges valued at nearly $70 billion.
The full agenda for the summit can be seen at www.clintonglobalinitiative.org/2012 (Editing by Eric Beech)
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