Chelsea Clinton, daughter of Sen. Hillary Rodham Clinton, D-N.Y., does a microphone check at the Democratic National Convention in Denver, Tuesday, Aug. 26, 2008. (AP Photo/Chris Carlson )

Chelsea Clinton, daughter of Sen. Hillary Rodham Clinton, D-N.Y., does a microphone check at the Democratic National Convention in Denver, Tuesday, Aug. 26, 2008. (AP Photo/Chris Carlson )

Chelsea Clinton reacts to applause after temporarily speaking for her mother, Democratic presidential hopeful Sen. Hillary Rodham Clinton, D-N.Y., after she had a coughing fit while speaking at a campaign event in Yankton, S.D., Monday, June 2, 2008. (AP Photo/Elise Amendola)

In this Wednesday, May 14, 2008 file photo, Chelsea Clinton visits the Luisa Guadalupe Center for the elderly, on the island of Vieques, Puerto Rico. Forget placards, stoic bodyguards and formal rallies. To win Puerto Rico's presidential primary, both the Clinton and Obama camps are campaigning in the boisterous, face-to-face "boricua style" favored on this Caribbean island. (AP Photo/Ricardo Figueroa)

Chelsea Clinton visits the Milivi Children's Center, on the island of Vieques, Puerto Rico, Wednesday, May 14, 2008. Clinton is campaigning in Puerto Rico for her mother, Democratic presidential hopeful Hillary Rodham Clinton, D-N.Y., ahead of the June 1 primary. (AP Photo/Ricardo Figueroa)

Chelsea Clinton visits the Luisa Guadalupe Center for the elderly, on the island of Vieques, Puerto Rico, Wednesday, May 14, 2008. Clinton is campaigning in Puerto Rico for her mother, Democratic presidential hopeful Hillary Rodham Clinton, D-N.Y., ahead of the June 1 primary. (AP Photo/Ricardo Figueroa)

Chelsea Clinton stands with supporters during a question and answer session with the public, at a campaign stop in San Juan, Puerto Rico, Tuesday, May 13, 2008. Clinton is campaigning in Puerto Rico for her mother, Democratic presidential hopeful Hillary Rodham Clinton, D-N.Y., ahead of the June 1 primary. (AP Photo/Brennan Linsley)

Chelsea Clinton waves to the crowd before the 134th Kentucky Derby Saturday, May 3, 2008, at Churchill Downs in Louisville, Ky. (AP Photo/Garry Jones)

Chelsea Clinton meets local politicians as she arrives to speak at Our Lady of Providence elderly home, in San Juan, Puerto Rico, Tuesday, April 29, 2008. She is spending two days here campaigning for her mother, Democratic presidential hopeful Hillary Rodham Clinton, D-N.Y., ahead of the June 1 primary. (AP Photo/Brennan Linsley)

Chelsea Clinton speaks to a group of residents at Our Lady of Providence elderly home, shortly after her arrival in San Juan, Puerto Rico, Tuesday, April 29, 2008. She is spending two days here campaigning for her mother, Democratic presidential hopeful Hillary Rodham Clinton, D-N.Y., ahead of the June 1 primary. (AP Photo/Brennan Linsley)