Senator Claire McCaskill (D) announced today that she will not be running for governor in 2016.

Senator Claire McCaskill (D) (left) and Attorney General Chris Koster (D) (right) [file photos].

Social media is all atwitter with the news:

Jason Hancock ‏@J_Hancock

“That’s a firm no.” — @clairecmc on idea of running for governor in 2016. Says she’s done no polling on the race, fwiw. #mogov #moleg 11:11 AM – 12 Jan 2015

Yael T. Abouhalkah ‏@YaelTAbouhalkah

Chris Koster is happiest guy in Missouri right now. @clairecmc stays in U.S. Senate. 11:12 AM – 12 Jan 2015

State Auditor Tom Schweich (r) is probably still pretty unhappy, though.

Jason Rosenbaum ‏@jrosenbaum

. @clairecmc says we have a “terrific candidate” in @Koster4Missouri. Can’t imagine he has significant opposition anymore. 11:12 AM – 12 Jan 2015

Maybe Leonard Steinman?

Eli Yokley ‏@eyokley

Why does @ClaireCMC support @Koster4Missouri? “So we can keep some sanity in Jefferson City among some pretty extreme folks.” #MOGov 11:12 AM – 12 Jan 2015

Some?

Tony Messenger ‏@tonymess

The most important thing @clairecmc can do for #MO right now has nothing to do with #mogov or #mosen: It’s pass an ethics ballot issue.11:14 AM – 12 Jan 2015

VirginiaYoung ‏@VirginiaYoung

.@clairecmc calls @Koster4Missouri “terrific candidate.” Q is whether she wrung any promises on what he’ll push in return for staying out. 11:16 AM – 12 Jan 2015

Ye of little faith.

Scott Charton ‏@ScottCharton

That wind isn’t just winter; it’s @Koster4Missouri exhaling as he buys the first round. 11:18 AM – 12 Jan 2015

daveweigel ‏@daveweigel

Are we so surprised that McCaskill didn’t want to trade a Senate seat for the right to have vetoes overridden by the MO lege? 11:23 AM – 12 Jan 2015

That is a good point.

Jane Dueker ‏@JaneDueker

Sorry #MOGOP your fantasy of a Democratic primary for Governor is over before it started. Time to focus on your own primary. 11:29 AM – 12 Jan 2015

Meanwhile, State Auditor Tom Schweich (r) has a sad.