Two Senate Republicans say they want the Treasury Department to explain why it gave a “blank check” to Fannie Mae and Freddie Mac by removing the $400 billion cap on how much money it would give the mortgage giants to keep them from failing. In a letter to Treasury Secretary Timothy Geithner, Sen. Bob Corker, R-Tenn., said he’s “concerned about the lack of understanding and transparency” surrounding Treasury’s Christmas Eve decision. Sen. David Vitter, R-La., joined the call for transparency, saying through a spokesman, “Given the size of the check Secretary Geithner just wrote to these two failed companies, he has a lot of explaining to do to Congress and the American people.”