As Twitchy reported earlier this week, Sen. Majority Leader Harry Reid (D-Nev.) tossed around unsubstantiated allegations about former Mass. Governor Mitt Romney’s unreleased tax returns. Apparently, somebody or other told Reid something or other about Romney’s taxes.

That was good enough for Sen. Reid, who repeated the rumor to the media and on the floor of the U.S. Senate.

After Romney told Reid earlier today to “put up or shut up,” Sen. Reid doubled down, insisting that his still-unnamed source is “extremely credible.”

Just who is Sen. Reid’s mysterious unidentified source? Reid’s not telling, but conservative Twitter users have some theories: