He makes a strong case for impeachment in today's Washington Post:
As we enter the eighth year of the Bush-Cheney administration, I have belatedly and painfully concluded that the only honorable course for me is to urge the impeachment of the president and the vice president.
After the 1972 presidential election, I stood clear of calls to impeach President Richard M. Nixon for his misconduct during the campaign. I thought that my joining the impeachment effort would be seen as an expression of personal vengeance toward the president who had defeated me.
Today I have made a different choice.