House Republicans are demanding an “equal playing field” in the Democrat-led impeachment probe against President Trump after Intelligence Committee Chairman Adam Schiff said ahead of Thursday’s scheduled testimony from former U.S. envoy for Ukraine Kurt Volker that GOP members of the Foreign Affairs Committee will not be permitted to ask questions or have equal representation during the session.

Volker is scheduled to be the first key witness to testify as part of a probe into an anonymous whistleblower’s complaint about a July 25 phone call in which Trump asked Ukrainian president Volodymyr Zelensky to investigate Democratic presidential candidate Joe Biden’s family’s dealings in the country. The interview will be behind closed doors.

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Kurt Volker -- until recently the U.S. special representative to Ukraine -- is seen in Kiev, Ukraine, Sept. 15, 2018. (Associated Press)

The lead Republican on the House Foreign Affairs Committee wrote Wednesday evening: “I was alarmed to learn – less than 24 hours before the first interview is scheduled to start – that it will be led by the Intelligence Committee and that questioning will be done solely by their staff.”

“We were told that only a single Republican professional staffer from the Foreign Affairs Committee will be allowed to attend while the majority will have two,” Rep. Michael McCaul, R-Texas, the committee’s ranking member, continued. “These constraints on committee and Republican participation are unacceptable and at odds with House Rules and general fairness. We demand equal representation and participation in this inquiry, there is too much at stake for America and Congress.”

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In the letter addressed to Chairman Eliot L. Engel, D-N.Y., McCaul further argued that despite statements made by House Speaker Nancy Pelosi, D-Calif., and other Democrats, “there is not a “House of Representatives’ impeachment inquiry” because the entire House has not voted on the matter.

Citing House Rules X and XI, McCaul said that until Congress members from both parties vote to create a special impeachment task force to carry out proceedings, “Intelligence Chairman Adam Schiff lacks the jurisdiction to investigate the Department of State’s conduct of United States foreign policy toward Ukraine. That prerogative belongs to our Members.”

“Official impeachment inquiries are initiated by the adoption of a House resolution empowering or creating a committee or task force to undertake such activities,” McCaul continued. “In both the Nixon and Clinton cases, the Judiciary Committee debated and reported a resolution authorizing the Judiciary Committee to investigate whether there were sufficient grounds to impeach the President, which was then debated and voted on by the full House of Representatives. There have been no such debates or votes in this Congress.”

On Thursday, Volker is expected to voluntarily give a transcribed interview before the Schiff-led Intelligence Committee as part of its impeachment inquiry into whether Trump abused his authority by asking Ukraine’s president to investigate Biden, a leading candidate for the Democratic Party's 2020 presidential nomination.

Volker resigned last week after his name surfaced in a whistleblower complaint alleging that Trump sought to pressure Zelensky to investigate Joe Biden, Hunter Biden, and Burisma, the Ukrainian energy firm that paid the former vice president’s son to serve on its board.

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Acting as the U.S. envoy for Ukraine, Volker met with Vadym Pozharskyi, an adviser to the board of directors of Burisma Group, in New York last year even as Trump’s private lawyer, Rudy Giuliani, was pressing Ukraine’s government to investigate the company and the Bidens’ involvement with it.

The Associated Press contributed to this report.