President Bush, moving toward a constitutional showdown with Congress, asserted executive privilege Thursday and rejected lawmakers’ demands for documents that could shed light on the firings of federal prosecutors.

Bush’s attorney told Congress that the White House would not turn over subpoenaed documents for former presidential counsel Harriet Miers and former political director Sara Taylor. Congressional panels want the documents for their investigations of Attorney General Alberto Gonzales’ stewardship of the Justice Department, including complaints of undue political influence.