1. She didn't read the classified 90 page NIE as she was supposed to read. Sen. Bob Graham begged the senators to read it before voting on the IWR. More about the NIE here.

2. She voted for the IWR/AUMF which was a blank check authorization for war and it's title reads: "A joint resolution to authorize the use of United States Armed Forces against Iraq." Floor speech (video)

3. She voted against the Levin amendment (which was a multilateral approach). See: The Senate’s Forgotten Iraq Choice

4. She voted against the Durbin amendment whose purpose was: "To amend the authorization for the use of the Armed Forces to cover an imminent threat posed by Iraq's weapons of mass destruction rather than the continuing threat posed by Iraq." Saddam was not imminent threat, and hence that amendment would have prevented a war.

5. On March 6, 2003, as UN inspectors were already reporting  by then (news reports were coming by late Feb, 2003) that no WMD or nuclear weapons/programs could be found, she was asking Saddam to "disarm", instead of demanding Bush not to invade until the inspections were completed 100% on which Blix was only asking for a couple of months to wrap up the inspections effort.

6. On March 17, 2003, two days before the invasion, despite the UN inspector reports, she demanded Saddam to accept Bush's "ultimatum" instead of telling Bush NOT to go in.

However hard she tries, she has no surviving excuse for her vote, in light of these facts. She voted for a war and it's inescapable that she wanted the war to take place.

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Furthermore, Hillary has a losing argument vs McCain (or for that matter Romney) on Iraq. He has already started making his argument.

McCain to HRC argument could sound like this: "Senator Clinton voted for the war when that was popular and started "opposing" it when the war became unpopular and she started running for the Democratic nomination. I risked everything and supported/promoted the "surge" when it and the war were highly unpopular, and we're prevailing as the surge has improved security in Iraq and put Al-Queda in Iraq on the run."

Obama has a winning argument on Iraq:

Obama to McCain argument could be like this: "We should never have gone in as I argued in Oct'02, and then we'd have 4000 troops alive today as well as we would have saved $1 trillion for the tax payers, would not have created a haven for Al-Queda in Iraq, and would not have lost some standing as a leader of the world."