WASHINGTON, July 9 — President Bush, invoking executive privilege for the second time in his clash with lawmakers over the firing of federal prosecutors, said today that he is refusing to comply with Congressional subpoenas for testimony from two top former aides.

In a letter to the chairmen of the House and Senate Judiciary Committees, Mr. Bush’s counsel, Fred F. Fielding, declared that the legislative and executive branches of government were at an impasse. Mr. Fielding wrote that the president is directing the two aides — Sara M. Taylor, the former White House political director, and Harriet E. Miers, the former White House counsel — not to testify.

“The assertion of executive privilege here is intended to protect a fundamental interest of the presidency: the necessity that a president receive candid advice from his advisors and that those advisors be able to communicate freely and openly with the president,” Mr. Fielding wrote, adding that in the case of the firing of federal prosecutors, “the institutional interest of the executive branch is very strong.”

The move was not unexpected. Mr. Bush said last month that he had no intention of letting Ms. Miers or Ms. Taylor testify.