Well, I can think of at least a couple of reasons: 1) Saddam Hussein, as the Democrat Party leaders said, was in violation of various UN resolutions.
2) Saddam Hussein, as the Democrat Party leaders said, was apparently busily developing WMDs.
3) None of the Democrat Party leaders said they were opposed to invading Iraq.
4) None of the Democrat Party leaders stepped forward and proposed we invade some other country instead of Iraq.
5) The Democrat Party leaders said they were concerned about the terrorist training camps in Iraq.
6) The Democrat Party leaders said they were concerned about the human rights violations that were occurring in Iraq.