Arlen's defection is huge news but rumor had it that John McCain very nearly defected towards the Democratic party a few years ago, instead opting to fight for the 2008 nomination. This was mentioned in Frontline's coverage of the race and in many other places including here:

http://thehill.com/leading-the-news/democrats-say-mccain-nearly-abandone...

So, is this unprecidented? A trend? Trully new? Discuss