‘Subpoena’ means ‘under penalty’ and the ‘separation of powers’ claim is just sand thrown into Waxman’s eyes. Waxman should take her to court, and have her arrested and jailed until she complies with the law, same as was done to Judith Miller.
(Rice would give in long before it reached that, and Republicans would want to retaliate if they somehow got control of Congress, but they do that stuff anyway, viz. Pete Domenici, because that’s who and how they are; the difference is that Waxman’s status would soar, while we know what happened to Newt Gingrich’s numbers.)