William Rivers Pit, always scrupulous about his information says this:
Right now, several other Senators are preparing statements of Support for the Boxer/Tubbs-Jones challenge, and a number of House members will also rise in support. There is every expectation that Senators Clinton, Obama and Dodd will be among those offering statements of support.KEEP THE CALLS COMING! THE MORE SENATORS SUPPORTING, THE MORE EFFECT YOUR CALLS CAN HAVE ON OTHERS—ESPECIALLY HARRY REID.Reps. Waters, Conyers and Kucinich will be among the House members who stand. Though Rep. Conyers was the main impetus behind this process, it was decided that Rep. Tubbs-Jones should be the one to make the official challenge, as she is a representative from Ohio, where the dispute is centered.
There is a rumor floating around that one of the Senators to rise in support will be a Republican. That is not in any way confirmed.