They didn't oppose it.
There was bipartisan support for the war.
"In the four years since the inspectors left, intelligence reports show that Saddam Hussein has worked to rebuild his chemical and biological weapons stock, his missile delivery capability, and his nuclear program. He has also given aid, comfort, and sanctuary to terrorists, including al Qaeda members ... It is clear, however, that if left unchecked, Saddam Hussein will continue to increase his capacity to wage biological and chemical warfare, and will keep trying to develop nuclear weapons."
- Sen. Hillary Clinton (D, NY), Oct 10, 2002 | Source
"I will be voting to give the President of the United States the authority to use force -- if necessary -- to disarm Saddam Hussein because I believe that a deadly arsenal of weapons of mass destruction in his hands is a real and grave threat to our security."
- Sen. John F. Kerry (D, MA), Oct. 9, 2002 | Source