I'll have a go at that...and say...momentum, persuasion, manipulation, and then a really nice one...doing what they believe will secure their seat in an upcoming election.
Have a look at the historical records of every publicly known war/confrontation ever since such things went 'formal'.
http://en.wikipedia.org/wiki/Declara..._United_States
Out of 24 declarations, 5, (Iraq, Afghanistan, Persian Gulf, Mexican American War, and the war of 1812- two of those two centuries ago), received any contention from opposed members of either the House or The Senate.
Food for thought eh.