Charles Dharapak, File, Associated Press
FILE - This Oct. 30, 2013, file photo shows David Simas, President Barack Obama’s political director, left, and Rob Nabors, Deputy Chief of Staff as they accompany the president to board Air Force One at Andrews Air Force Base, Md. The White House is opposing congressional Republicans’ subpoena to Simas to testify on Capitol Hill this week. White House counsel Neil Eggleston said in a letter that the House Oversight Committee’s demand that Simas testify threatens the president’s independence and ability to get candid advice to carry out his constitutional duties.

WASHINGTON — The White House is defying a congressional subpoena by keeping a top political aide to President Barack Obama from testifying at a hearing Wednesday.

The White House is arguing that Obama adviser David Simas (SEE'-muhs) is immune from subpoena by the House Oversight Committee in its investigation into the political office that he directs.

White House counsel Neil Eggleston says in a letter to Oversight Chairman Darrell Issa (EYE'-suh) that the Simas subpoena threatens the constitutional separation of powers.

White House lawyers spent an hour and 15 minutes Tuesday briefing the committee's staff behind closed doors on the political office's efforts to comply with federal law. But Issa says the committee still has questions he wants Simas to answer publicly.