From "Rice Defends Refusal To Testify" by Dana Milbank and Walter Pincus, in the March 29, 2004, edition of the Washington Post:

Rice gave no ground on the administration's decision that she will not appear in public before the panel or testify under oath because Bush officials believe doing so would compromise the constitutional powers of the executive branch. The renewed refusal came despite the panel's unanimous plea for her testimony.

Republican commissioner John F. Lehman, who has written extensively on separation-of-power issues, said that "the White House is making a huge mistake" by blocking Rice's testimony and decried it as "a legalistic approach."

"The White House is being run by a kind of strict construction of interpretation of the powers of the president," he said on ABC's "This Week." "There are plenty of precedents that the White House could use if they wanted to do this."

[...]

Rice said she has "absolutely nothing to hide" and "would really like" to testify but will not because of the constitutional principle.

Gee, guys, this whole "Constitution" document sure comes in handy when you need it most, huh? That is, when you're not too busy covering your ears to cries of "Hypocrisy!" and otherwise obliterating the fucking thing, like you've been doing for the past two-and-a-half years.

RELATED (and very much worth reading): Josh Marshall at Talking Points Memo discusses the issue of Constitutional precedent here and here.