Sen. Mark Kirk: I'm Un-Endorsing Trump

And the un-endorsements of Donald Trump have begun, starting with Illinois Sen. Mark Kirk:

First un-endorsement -- Sen. Mark Kirk. "I cannot and will not support my party's nominee for President."

— Erica Werner (@ericawerner) June 7, 2016

Sen MARK KIRK, perhaps most endangered gop incumbent, says no to Trump

"I cannot and will not support my party's nominee for President"

— Jonathan Martin (@jmartNYT) June 7, 2016

Kirk said that he does not think Trump is fit to control the nuclear arsenal:

Given my military experience, Donald Trump does not have the temperament to command our military or our nuclear arsenal.

— Mark Kirk (@MarkKirk) June 7, 2016

Back in March, Kirk confirmed that he would definitely support the Republican nominee, even if it were Trump.

Kirk is considered to be a very vulnerable Republican senator, and trails Democratic challenger Rep. Tammy Duckworth in several polls.

Buckle up folks, it's only going to get more interesting from here on out...