In 2008, McCaskill supported President Obama over Clinton in the Democratic primary.
"Hillary Clinton had to give up her political operation while she was making us proud, representing us around the world as an incredible secretary of state, and that's why Ready for Hillary is so critical," McCaskill said in a statement provided by ReadyforHillary.com.
Clinton recently joined Twitter, opening an account in which she hints at future job prospects, Politico reported.
Her Twitter biography reads "Wife, mom, lawyer, women & kids advocate, FLOAR, FLOTUS, US Senator, SecState, dog owner, hair icon, pantsuit aficionado, glass ceiling cracker, TBD."
Senate Democrats to pull all-nighter on climate change
Interpol investigating stolen passports on missing flight