Clinton was often Obama's harshest Democratic critic, trying to bring down the rival as his candidacy surpassed Hillary Rodham Clinton's.

While Hillary Clinton has begun to help Obama by encouraging her supporters and fund-raisers to back his campaign, a chill remained between the last Democratic president and the man running to be the next one.

Obama spokesman Bill Burton said the Illinois senator was honored to have the former president's support.

"He has always believed that Bill Clinton is one of this nation's great leaders and most brilliant minds, and looks forward to seeing him on the campaign trail and receiving his counsel in the months to come," Burton said.

Clinton spokesman Matt McKenna said the former president renewed his offer, expressed in a one-sentence statement last week, to do whatever he could to ensure Obama wins the presidency.

"President Clinton continues to be impressed by Sen. Obama and the campaign he has run, and looks forward to campaigning for and with him in the months to come," McKenna said. "The president believes that Sen. Obama has been a great inspiration for millions of people around the country, and he knows that he will bring the change America needs as our next president."

Obama spokesman Robert Gibbs said the senator called Clinton after Obama landed in Missouri yesterday morning, and they spoke for about 20 minutes. Gibbs said Obama asked Clinton to campaign with him and on his own.

Bill Clinton was in Europe last week and did not attend Friday's rally with his wife and Obama in the symbolic town of Unity, N.H. Obama said it was appropriate that he appear alone with his former rival, since they had waged a historic race.

Bill Clinton was an outspoken critic of Obama during the primaries. He said Obama's opposition to the Iraq war was a "fairy tale" and raised questions about whether the first-term Illinois senator had the experience to lead the country. During one debate Obama told Hillary Clinton: "I can't tell who I'm running against sometimes."