The February filibuster of Chuck Hagel was the last straw for Harry Reid.
In the eyes of the Democratic majority leader, Republicans had just shattered a mere three-weeks-old agreement to avoid a drastic change to the Senate rules. Not only was the Hagel vote the first time in U.S. history the Senate filibustered a nominee to be secretary of defense, but the minority party took an unprecedented step and rebuked one of their own: Hagel was a former red state Republican senator who also happened to be a decorated war veteran.
In the days that followed, a few things crystallized in the Nevada senator’s mind. The re-election of President Barack Obama and Republican promises of a "new day" in the Senate were all for naught. It became clear that the GOP was not going to change its practice of obstruction, which was rendering dysfunctional not only the legislative branch but also the executive and judicial branches by thwarting presidential nominations.