Facing a tough reelection against Congresswoman Tammy Duckworth, vulnerable GOP Senator Mark Kirk of Illinois announced today that he is un-endorsing Republican presidential nominee Donald Trump. In a Tweet, Kirk said his military experience helped lead him to make the decision:Given my military experience, Donald Trump does not have the temperament to command our military or our nuclear arsenal.  Mark Kirk (@MarkKirk) June 7, 2016Then, in a statement, Kirk told reporters:  As the presidential campaign progressed, I was hoping the rhetoric would tone down and reflect a campaign that was inclusive, thoughtful and principled. While I oppose the Democratic nominee, Donald Trump s latest statements, in context to past attacks on Hispanics, women, and the disabled like me, make it certain that I cannot and will not support my party s nominee for President regardless of the political impact on my candidacy or the Republican Party. And like that, Kirk became the first Republican to un-endorse Donald Trump. Benn Sasse and Lindsey Graham, Republican Senators from Nebraska and South Carolina, have pledged not to support Trump either, and Trump s former rivals, Senators Ted Cruz and Marco Rubio are still on the fence.Other Republican Senators, such as Kelly Ayotte, Bob Corker, Mitch McConnell, Susan Collins, and Jeff Flake, have all expressed condemnation for Trump s comments about the judge presiding over the University case, but have pledged continued support.Kirk s announcement, coincidentally, comes just one day after his challenger, Duckworth, said Kirk s refusal  silence  equals  betrayal  to the people.Watch Kirk explain to reporters his un-endorsement: JUST IN: @SenatorKirk talks exclusively to @nbcnews  @HallieJackson RE: him not supporting Trump: pic.twitter.com/OsxSmkwDt0  Frank Thorp V (@frankthorp) June 7, 2016Featured image via Gabriella Demczuk