U.S. Sen. Claire McCaskill, D-Mo., wants Hillary Clinton to run for president in 2016 and on Tuesday joined a group of activists hoping to make that happen.

The pro-Clinton Super PAC Ready for Hillary announced that McCaskill, who backed President Obama in 2008 over Clinton, was behind the former first lady and secretary of State this time around.

"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. "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 is the first member of Congress to sign on to the effort, which is made up largely of Clinton friends and advisers, including Democratic strategist James Carville. For her part, Clinton has yet to say whether she will seek the office again after her failed 2008 bid.

Ready for Hillary launched in January, just eight days after Obama was inaugurated for his second term.