Obama takes aim at last rival standing

"When I am the nominee, the Republicans won't be able to make this election about the past," he told an overflow crowd of more than 10,000 at the University of Denver in a speech filled with sharp jabs at his rival.

"That's what the Republicans are going to want to do," he said. "They are going to want us to look backwards, but they won't be able to do that with me. Because you will have already chosen the future."

Clinton aides were quick to swat back at Obama for characterizing the New York senator and former first lady as a polarizing figure and emblematic of an outdated style of politics. "His words in that speech were a summary of all the negative attacks, almost a 'greatest hits' of negative attacks he has launched throughout this campaign," Mark Penn, Clinton's pollster and campaign strategist, said in a conference call with reporters. "They are false. They are personal. They are unwarranted."

Speaking in the city where the Democratic Party will hold its nominating convention seven months from now, Obama started out by praising the "all-star cast" of candidates who have run for the party's presidential nomination this year, all but one of whom has left the race: Sen. Christopher J. Dodd; Sen. Joseph R. Biden Jr.; Rep. Dennis J. Kucinich; Gov. Bill Richardson; former Sen. John Edwards, who dropped out Wednesday; and Clinton.

But that was pretty much where the Illinois senator's praise of Clinton ended. He spoke of how Democrats could win in November and build on its majority in Congress: "not by nominating a candidate who will unite the other party against us, but by choosing one who can unite this country around a movement for change."

His barbs never mentioned his rival by name, but Obama took a direct slap at Clinton's past divisiveness and her failure to achieve healthcare reform as first lady or senator.

A mother with a sick child and no insurance "can't afford to wait another four years -- or another 15 years -- to get healthcare because we've put forward a nominee who can't bring Democrats and Republicans together to get it done," he said.