Stressing that "it's important we start early," McCaskill today threw her support behind Clinton, who has barely caught her breath following four years as secretary of state.
"Now, as I look at 2016 and think about who is best to lead this country forward, I'm proud to announce that I am Ready for Hillary," the Missouri Democratic lawmaker said in a statement posted on the website ReadyforHillary.Com.
In 2008 McCaskill was an avid supporter of Barack Obama, who bested Clinton in the Democratic primary on his road to the White House, something the senator acknowledged in her statement.
Clinton, who was first lady when Bill Clinton was in the White House, has inched back out into the spotlight in recent weeks, announcing plans to work on global initiatives involving women and early childhood.
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Within a week she gained more than 500,000 Twitter followers.
McCaskill said "it's important that we start early, building a grassroots army from the ground up, and effectively using the tools of the Internet" to help Clinton win if she decides to run.
Poll after poll shows Clinton well ahead of any potential Democratic challengers for the White House, including Vice President Joe Biden.
But recent surveys also show her favorability taking a hit, with the controversy over the attack on the US mission in Benghazi, Libya last year a likely factor.