There are 927 days between today and Jan. 1, 2016. And yet, Missouri Sen. Claire McCaskill chose today to announce her support for former Secretary of State Hillary Clinton's potential presidential bid in three years time.

Missouri Sen. Claire McCaskill endorses Hillary Clinton for president today. AP Photo.

“It’s important that we start early, building a grassroots army from the ground up, and effectively using the tools of the Internet – all things that President Obama did so successfully – so that if Hillary does decide to run, we’ll be ready to help her win," McCaskill said in a release sent Tuesday morning from "Ready for Hillary", a super PAC supporting a Clinton bid in 2016.

The question isn't why McCaskill endorsed Hillary -- there's no one else you would even think about endorsing at this point -- but rather why she did it now. Here's two possible reasons:

1. McCaskill was one of the leading -- and most effective -- spokespeople for then Illinois Sen. Barack Obama in 2008. That was a good thing for McCaskill then, but less so now -- particularly because of her history with the Clintons. Remember that in the fall of 2006, McCaskill went on "Meet the Press" and said this about Bill Clinton: "I think he’s been a great leader but I don’t want my daughter near him." Oomph. By endorsing Clinton so early, McCaskill is saying "I wasn't with you then but I am, big time, now."  She's trying to heal the wound created by her endorsement and advocacy of the man who beat Clinton five years ago.

2.  McCaskill knows that her political future in Missouri is a difficult one. She almost certainly would not be in the Senate right now if Republicans hadn't nominated Todd "legitimate rape" Akin in 2012. Missouri has moved from a swing state to a GOP-leaning one in the last decade and there's no reason to believe, demographically speaking, that it will move back toward Democrats any time soon.

That means that McCaskill will face a VERY difficult race in 2018. She knows that and may well be looking at her other political options -- one of which would be to serve in a Clinton Cabinet.  The best way to set yourself up for that possibility is to pick a candidate as early as possible -- see Tim Kaine's endorsement of Obama in 2007 -- and then work like hell to try to get them elected.

It's also worth noting when analyzing McCaskill's calculation that she undoubtedly believes that Clinton is the person best equipped to handle the rigors of the presidency in the 2016 field. But, politicians never -- repeat: never -- do things without thinking of the impact on them and their careers. And McCaskill is a politician.