Guardian: President Barack Obama will tomorrow night make his first address to the American people from the Oval Office, underlining the scale of the catastrophe now unfolding in the Gulf of Mexico and vowing to hold BP to account for the environmental damage wrought by its eight-week-old oil spill. His prime-time broadcast comes at the start of a crucial week in the response to the crisis. Senior politicians from the Obama administration and the Democratic hierarchy in Congress are lining up …