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Bush has established the principle...
In a letter to the panel, the White House sought written assurances that Rice’s testimony would set no precedent and that no more public testimony from any White House official would be requested.
The commission accepted the terms, saying in its response that “Dr. Rice’s appearance before the Commission is in response to the special circumstances presented by the events of September 11 and the Commission’s unique mandate.”...
The White House and Rice had maintained that requiring a national security adviser to testify under oath would compromise “executive privilege,” which allows a president to exchange ideas freely with an adviser without fearing that they would be made public.
“A president and his advisers, including his advisers for national security affairs, must be able to communicate freely and privately without being compelled to reveal those communications to the legislative branch,” Bush said.
“We have observed this principle while also seeking ways for Dr. Rice to testify,” he added. http://www.msnbc.msn.com/id/4623066/print/1/displaymode/1098/ (Via POGO blog - http://pogoblog.typepad.com/pogo/2007/03/executive_privi.html)
There's an interesting account of Bush's claim of executive privilege here: http://www.law.duke.edu/adminlaw/execpriv.pdf pages 51-57. Bush refused to turn over DoJ documents to the House Government Reform Committee in 2001-2002:
The resolution of this controversy was somewhat reminiscent of many former executive privilege battles, especially the ones during the Reagan years. In each of those battles the administration staked out a strong stand on executive privilege and signaled a refusal to compromise; Congress persisted and used its authority to pressure the administration to turn over the disputed materials; the administration ultimately relented on either all the documents, or at least the key ones; both sides walked away and declared victory [but it took a lot of time, of course].