The way I heard Biden-Lugar explained on the Dean blog, it required Bush to certify there were WMD in Iraq before invading. If he certified WMD had been proven, then invaded and no WMD were found (which is what happened), that would have been impeachable. That is, instead of talking about sending Bush back to Crawford, we'd be talking about sending him to jail. It also explicity forbid Bush from hopscotching from Iraq onward to other mideast countries the way he apparently planned to do before the Iraq operation got bogged down.
At that time, UN inspectors were still in Iraq and it was very plausible that they might find something. Biden-Lugar was an entirely appropriate thing to support. The blank check that Bush got instead was an outrage.