"We write in order to urge that you require your Deputy White House Chief of Staff, Karl Rove, to either come forward immediately to explain his role in the Valerie Plame matter or to resign from your Administration," the veteran Democrat writes. "High-ranking members of your Administration who are involved in any effort to smear a private citizen or to disseminate information regarding a CIA operative should be expected to meet a far higher standard of ethical behavior and forthrightness."