By September of 2003, a guerrilla war was raging in Iraq and it had become clear that there was no WMD. Shiite cleric Muhammad Baqir al-Hakim was blown up in Najaf on August 31. GIs were being killed in al-Anbar.
Bush and Cheney needed to refute Wilson's allegation that they ignored his report on Niger uranium. They also needed a smoking gun to tie Iraq to al-Qaeda, without which their continued occupation of the country was on thin legal grounds.
Tying Atta to Abu Nidal would form an ex post facto justification for the war, something Bush desperately needed.