The president has refused to let his aides testify under oath before a Senate panel investigating his Justice Department, saying, "There is no indication that anybody did anything improper."
That is not for this, or any president, to decide. It is the Senate's proper and constitutional role to investigate and collect the best evidence. This was all settled by the Supreme Court's unanimous vote against Richard Nixon's stonewall in 1974. Now, with President Bush's refusal to cooperate, it's very clear that he has something to hide.
As far as I'm concerned, impeachment is back on the table.