Democratic presidential candidate Barack Obama on Saturday backed away from rival John McCain's challenge for a series of joint appearances, agreeing only to the standard three debates in the fall.Obama's reversal on town hall debates is part of a play-it-safe strategy he has adopted since claiming the nomination and grabbing a lead in national polls. Advisers to the Illinois senator, speaking on condition of anonymity because they are not authorized to discuss strategy, say Obama is reluctant to take chances or give McCain a high-profile stage now that Obama is the front-runner.In May, when a McCain adviser proposed a series of pre-convention appearances at town hall meetings, Obama said, "I think that's a great idea." In summer stumping on the campaign trail, McCain has often noted that Obama had not followed through and joined him in any events.On Saturday, in a letter to the Commission on Presidential Debates, Obama campaign manager David Plouffe said the short period between the last political convention and the first proposed debate made it likely that the commission-sponsored debates would be the only ones."We've committed to the three debates on the table," campaign spokeswoman Jen Psaki said Saturday in an interview. "It's likely they will be the three appearances by the candidates this fall."Asked by The Associated Press if that meant Obama would not agree to any other debates, Psaki said, "We're not saying that." She said the McCain campaign had rejected Obama's proposal for two joint town hall meetings.McCain's campaign disparaged Obama for backing off. "We understand it might be beneath a worldwide celebrity of Barack Obama's magnitude to appear at town hall meetings alongside John McCain and directly answer questions from the American people, but we hope he'll reconsider," spokesman Brian Rogers said.The first debate planned by the commission is set for September 26 in Oxford, Mississippi, three weeks after the Republican National Convention concludes September 4. The Democratic convention is scheduled for August 25-28.The other presidential debates are set for Oct. 7 and October 15 and the vice presidential debate for October 2.A day after Obama clinched the Democratic nomination in early June, McCain challenged Obama to a series of 10 town hall meetings. The candidates' campaigns began negotiations, telling reporters that they agreed in spirit to joint appearances.When the idea first came up from the McCain campaign that May, Obama was still battling Hillary Rodham Clinton for the Democratic nomination. Obama said then: "Obviously, we would have to think through the logistics on that, but ... if I have the opportunity to debate substantive issues before the voters with John McCain, that's something that I am going to welcome."In June, Plouffe had suggested Obama-McCain meetings more along the lines of the historic Lincoln-Douglas debates. During Abraham Lincoln's Senate campaign against Stephen Douglas in 1858, the candidates met seven times across Illinois. One spoke for an hour, the other for an hour and a half, and the first was allowed a half-hour rebuttal.Plouffe said Saturday that Rep. Rahm Emanuel of Illinois will be Obama's representative in further discussions with the commission.The Commission on Presidential Debates, established in 1987, sponsors and produces debates featuring the presidential and vice presidential candidates of the major parties. The nonprofit and nonpartisan organization has sponsored all the presidential debates since 1988.