"But what you have to ask yourself is who has the chance to actually really change things in a fundamental way so that 10 years from now or 20 years from now you can look back and you can say, boy, we really moved in a new direction and we put the country on a better path," Obama added as he wrapped up an event at Reading High School.
Obama was trying to argue that he is the better choice over Democratic rival Hillary Rodham Clinton in Tuesday's primary in Pennsylvania. But the Illinois senator ended up mixing in praise for McCain at the same time - and giving Clinton an opening to criticize.
"We need a nominee who will take on John McCain, not cheer on John McCain," the New York senator said in Johnstown.
Obama's comment threatened to undercut his efforts - and those of the entire Democratic Party - to portray the GOP presidential nominee-in-waiting as nothing more than an extension of Bush's unpopular tenure. At the very least, it provided fodder that Republicans can use in the general election to prop up McCain.
Earlier, Obama renewed his criticism that McCain offers the same "failed" policies of the Bush administration on everything from Iraq to the economy.
"We cannot afford a third George Bush term and that's what John McCain is offering - a third Bush term," Obama said, repeating a line he's said at virtually every campaign stop since McCain wrapped up the GOP nomination last month.
Democrats are trying to make the general election a referendum on Bush, whose job approval ratings have been dragged down by the Iraq war and a troubled economy.
McCain, for his part, is casting himself as an independent thinker with his own vision for the country and a record of fighting the status quo in nearly three decades in Washington.
Said Tucker Bounds, a McCain spokesman: "The remark underscores that John McCain has the strength to change America and move this nation forward. Barack Obama is a new face who represents old ideas."
Countered Obama spokesman Bill Burton: "It's hard to imagine a president doing a worse job than President Bush but one thing is clear, John McCain wants to do his best to emulate Bush's failed economic and foreign policies and even his divisive political tactics."
