(1) The resolution was non-binding and substantively different from the 2002 use of force resolution. Lieberman-Kyl has zero authority; it expresses senses, not findings.
(2) Obama's main substantive argument is about power and authority: should Congress give President Bush even a hint -- and Obama, looking at the intent of the initial sponsors, sees such a hint -- that the commander in chief has their permission to launch military strikes or continue to justify a large troop presence in Iraq on the basis of Iran's intervention in the war?
(3) Clinton's main argument is that she is a senator, and according to Obama's logic, anything that comes up for a vote could be interpreted or warped so as to give President Bush's actions the imprimateur of legitimacy. Clinton is on the Armed Services Committee; Iran's Revolutionary Guards are attacking U.S. soldiers; the biggest beneficiary of the Iraq war being ran, it's simply reality -- as Obama himself acknowledged -- that Iran's actions will in some way factor into decisions about when and where and how fast to move troops out. Clinton turned a sketchy resolution into a much better one, one that wasn't bellicose and gave no hints or winks to the White House. And, oh -- Obama didn't show up and argue this during the vote itself.