That put Democratic officeholders in the position of antagonizing either their antiwar party base or the majority of voters. Democratic leaders such as Hillary Clinton, Joe Biden, John Kerry and (remember him?) John Edwards chose to go with majority opinion -- and came to rue it when unknown antiwar candidates such as Howard Dean and, later, Barack Obama shot upward in presidential primary polls.
So when things in Iraq started to go badly, they were happy to suggest that they had been deceived into voting for war by an increasingly less popular president.
Another reason for the strength of the "Bush lied" meme is that Bush and his appointees did not effectively defend themselves against the charge.
Bush's press secretary repudiated the "sixteen words" in a Bush speech that simply recited a British intelligence conclusion, which the Brits never withdrew. And the Bush people did not push back on the ridiculous claim by former diplomat Joseph Wilson that he had proved Saddam Hussein's regime never sought WMD materials in Africa.
Nor was Team Bush successful in refuting the charge that it had violated espionage law by revealing the identity of Wilson's wife, Valerie Plame. The press took up the charge with glee and the administration official who could have refuted it, Deputy Secretary of State Richard Armitage, chose to stay mum.
In the current debate on Syria, some lawmakers, notably Democratic Rep. Alan Grayson, have expressed skepticism about intelligence conclusions that Bashir al-Assad's regime used chemical weapons.
They point to the unlikely possibility that Assad opponents could have done so and demand the kind of evidence of guilt beyond a reasonable doubt that is rightly required in domestic criminal trials. But it's obviously impossible to provide such evidence in a war zone like Syria.
In foreign affairs, decision-makers must act on imperfect information and on the basis of probabilities. They must keep in mind a lesson taught by history, that the course of military action is always unpredictable and subject to sudden reversal.
There are serious arguments for and against authorizing military action in Syria. The argument that the allies were lied into war in Iraq is not one of them.