Since Romney voted yes to Obstruction of Congress, he now needs to be impeached for Obstruction of the Executive. After all, they are co-equal branches of government.
If Romney believes that a member of one branch of the federal government can be impeached over lack of cooperation with another branch, let him be held to that same standard.
Romney voted not guilty on Obstruction of Congress. And a Senator is not a “civil officer of the United States” subject to impeachment.