During the 2006 mid-term campaign, Nancy Pelosi pledged that a Democratic victory would not mean a drive to impeach the President (and, by implication, the Vice-President). Much of the call for impeachment was based upon allegations of lying to Congress and the American people about the pre-war intelligence, and violations of the oath of office to preserve, protect and defend the Constitution when they knowingly violated laws and Constitutional provisions protecting individual rights and liberties. Weighty stuff, to be sure, but taken off the table by the Speaker-to-be's pledge, based upon what was known then.
As indicated in Libby Trial Shows Cheney Provided Aid-and-Comfort to Enemies of the United States, Libby and Rove could easily have said that they told reporters not knowing that Valerie Plame was undercover, but that would have pointed the finger at Dick Cheney as the person who learned of Plame's identity, and (whether properly declassified or not) decided to out her and her operation. To protect Cheney, Libby and Rove conceived the alibi that they heard about Plame from reporters and, believing the reporters would not testify to their conversations, thought they had created a perfect circle.
The Libby Trial, therefore, presents information that it was Dick Cheney provided aid-and-comfort to enemies of the United States. Unlike the above-mentioned transgressions of the Bush Administration, that would be defended as "political" judgments (and they covered themselves with rogue legal opinions on torture, habeas corpus violations and illegal searches), Cheney's act is specific, concrete and traitorous. It is also new, outside the scope covered by the pre-election pledge.
The Congress cannot ignore it.
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