WASHINGTON (AP) — The White House is opposing congressional Republicans' subpoena of a top political adviser to President Barack Obama to testify this week, saying the demand threatens the constitutional separation of powers.

White House counsel Neil Eggleston sent a letter Monday to House Oversight Committee Chairman Darrell Issa, asking him to lift to the subpoena to David Simas, director of the White House Office of Political Strategy and Outreach. But Issa's spokeswoman says Simas was still expected to appear at Wednesday's hearing, which will examine whether the White House political office is an abuse of taxpayer funds.

Administrations of both parties have maintained in-house political shops at the White House. A 2011 report by the independent Office of Special Counsel criticized a longstanding practice by both parties of using the political office for systematic, campaign-related activity.

Obama closed the White House Office of Political Affairs in 2011, but reinstated a political office under the new name earlier this midterm election year, slimmed down from the previous 15 staffers to six, with Simas in charge. Issa, R-Calif., has been demanding that the White House turn over all documents relating to the reopening of the office.

"Chairman Issa has allowed that if the White House provided the requested information to the committee, he would reconsider, but at present Mr. Simas is still expected to appear at Wednesday's hearing," Issa spokeswoman Becca Glover Watkins said.

Eggleston said his staff was planning to brief Issa on Tuesday on the political office's compliance with the law. Eggleston objected to the subpoena coming three business days before the hearing and pointed out that administrations of both parties have long argued that a congressional demand for presidential advisers to testify "raises significant separation of powers concerns."

"The committee's effort to compel Mr. Simas' testimony threatens longstanding interests of the executive branch in preserving the president's independence and autonomy and his ability to obtain candid advice and counsel to aid him in the discharge of his constitutional duties," Eggleston wrote. "Your decision to pursue this course of action is precipitate and surprising in light of our clear willingness to work with you to meet your informational needs and the fact that you have not pointed to any evidence that OPSO has violated the Hatch Act."

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