The New York senator spoke on NBC News' "Meet the Press."
Moderator Tim Russert said Clinton has voted for funding at least 10 times in the past, but the senator said she had reached a turning point.
"I voted against funding last spring," she said. "I understand that we're going to have a vote shortly about funding, and I will vote against it because I think that it's the only way that we can demonstrate clearly that we have to change direction."
Clinton has had to fight conservative criticism that she has flip-flopped on the war. She and other Democrats joined Republicans to give President Bush authority to prosecute the war before the 2003 invasion.
"Now, obviously, if I had known then what I know now about what the president would do with the authority that was given him," she said, "I would not have voted the way that I did, but the real question before us today is what do we do, going forward?"
She also praised U.S. Army Gen. David Petraeus, the overall Iraq commander, further distancing herself from criticism by the liberal group MoveOn.org.