Obama, accusing the New York senator of trying "to rewrite history," said Clinton still contends that her 2002 vote authorizing military intervention in Iraq was "not really a vote for war."
"She cast her vote after failing to read the national intelligence estimates on Iraq," which raised doubts in some lawmakers' minds about the justification of ousting Saddam Hussein, Obama said during a discussion with armed service members.
Obama, who was in the Illinois Senate at the time, publicly opposed the invasion.
"We need accountability in our leaders," said Obama, now a U.S. senator from Illinois. "You can't undo a vote for war just because the war stopped being popular."
Obama has been campaigning across South Carolina this week ahead of the state's Democratic presidential primary on Saturday.
A poll released Thursday evening showed Obama still leading Clinton 38 percent to 30 percent, but his support among men had dropped by half in just a week. Obama is supported by 10 percent of likely male voters now, and the rest of his male support shifted to John Edwards, according to the McClatchy-MSNBC poll.
Obama said voters "need to judge us on the judgments we've made and the lessons we've learned." Instead of saying her vote was wrong, Obama said, Clinton "has simply blamed the civilian and military leaders who carried out the policy she authorized."
Clinton has said her vote was intended to strengthen President Bush's hand in building international pressure against Hussein, not to approve a prompt invasion.
Zac Wright, a Clinton campaign spokesman in South Carolina, said it was "tough to take his comments too seriously."
"There hasn't been much action to match Senator Obama's talk," Wright said.
Both Hillary Clinton and Obama have voted against legislation that paid for the war but lacked a timetable for troop withdrawal. Obama has supported withdrawal of combat troops in 16 months.
