The long shadow of Iraq.
However skillfully Hillary Clinton tries to justify her decision to vote for Iraq war she will eventually be forced to pay a price for her refusal to admit the error. Democratic candidates who have either been always against the war (Obama) or subsequently admitted mistake (Edwards) are going to have the edge in primaries. (This is of course not such a major issue for Republicans as their base consists of one-time supporters of Iraq war.)
It is misleading to say that the nation was gripped by a post-9/11 shock and that there was no other option. It overlooks the fact that fully 23 Senators voted against the resolution (almost a quarter). In any case, the Senators are supposed to be independent and not susceptible to the general emotions of the moment.
Another important thing was that there were indeed other options. As the op-ed Senate's Forgotten Iraq Choice by Lincoln Chafee confirms there was a very concrete alternative option that was available at the time. Chafe was a Republican Senator from Rhode Island who lost the election last November. He is very liberal and has been a consistent critic of Bush and did not vote for him in the 2004 election.
This alternative was a resolution called Multilateral Use of Force Authorization Act of 2002 proposed by Carl Levine, a Michigan Democrat. It suggested a more multilateral strategy for the US. While agreeing that Iraq was a threat, it made the important point which seemed to have escaped many at the time, that it was far from an imminent threat.
Of course the resolution failed. But as Chafee remarks, it was "incomprehensible" why it received only 24 votes. Many Democrats (including Clinton and Edwards) and all Republicans other than Chafee voted against it.
The simple fact is that bellicose nut jobs like Cheney and Rumsfeld had decided right from the beginning of the Bush administration that they had to remove Saddam. Before 9/11 they were more interested in Iraq than serious reports from CIA about al-Qaeda. 9/11 gave them a great chance. They skillfully built up the case for Iraq war and marketed it brilliantly. All this is established wisdom now. For instance, Frank Rich lays it out nicely in his book. And many big name Democrats fell for this very easily. No amount of clever word play will change this simple fact.