WASHINGTON — The top U.S. diplomat in Ukraine who questioned why military aid for Kyiv had been held up by the White House has hired a veteran Republican attorney to help him navigate a request from House Democrats to testify in their impeachment inquiry, according to a letter obtained by NBC News and what sources familiar with the matter told NBC News.

Bill Taylor, who came out of retirement to serve as charge d'affaires in Kyiv in June, will be represented by John Bellinger, who served as a senior official in President George W. Bush's administration, including stints at the National Security Council and as the State Department's top lawyer, the sources said.

Taylor is expected to testify Tuesday.

Bellinger is also representing P. Michael McKinley, the former senior adviser to Secretary of State Mike Pompeo who testified Wednesday, the sources said. Bellinger accompanied McKinley as he arrived on Capitol Hill for the deposition, which was held behind closed doors.

Bellinger, partner with the Arnold and Porter law firm, was one of a number of prominent "Never Trump" Republicans in the foreign policy establishment who publicly opposed Donald Trump's candidacy in 2016. He drafted an August 2016 letter signed by 50 senior officials, many of them Republicans, warning of the dangers of electing Trump as president, and predicting he would be the "most reckless President in American history."

Bellinger has since said that every word of the letter "has turned out to be true, and worse."

Ambassador William Taylor in Kyiv, Ukraine, on July 19, 2019. Barcroft Media / Barcroft Media via Getty Images

Colleagues of both Taylor and McKinley described Bellinger, with his experience in the White House, the State Department and national security law, as uniquely qualified to represent diplomats in the Ukraine inquiry.

Apart from Bellinger, Taylor’s legal team includes Jeffrey Smith, a former CIA general counsel during the Clinton administration with extensive experience in national security legal issues.

Taylor has emerged as a key witness based on recently released text messages between him and the U.S. ambassador to the European Union, Gordon Sondland, and the former U.S. special envoy to Ukraine, Kurt Volker. Taylor expressed his concern about where the administration was headed in its approach to Ukraine's newly elected President Volodymyr Zelenskiy, warning against tying a White House meeting or U.S. military assistance to investigations meant to benefit Trump's re-election effort.

"As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign," Taylor wrote to Sondland on Sept. 9, according to text messages released by the congressional committees conducting the impeachment inquiry.

The confrontation between the executive branch and Congress over the impeachment inquiry has put Taylor and other diplomats in a difficult bind.

The White House has blasted the impeachment inquiry as a politically motivated attack on the president without legal justification, and refused to cooperate. The State Department has ordered diplomats not to appear before House lawmakers for depositions.

But after House Democrats issued subpoenas, four diplomats chose to cooperate. Former U.S. ambassador to Ukraine, Marie Yovanovitch, whose tenure was cut short by President Trump, appeared on Friday, George Kent, deputy assistant secretary in the Bureau of European and Euroasian affairs, appeared on Tuesday and McKinley, who resigned last week as Pompeo’s senior adviser, appeared on Wednesday. The U.S. ambassador to the European Union, Gordon Sondland, a political appointee, is due to testify on Thursday.

In her opening statement, Yovanovitch said she was recalled from her post at the request of the president even though one of her bosses told her she had done "nothing wrong." She expressed dismay that the U.S. government would remove an ambassador based, "as best as I can tell, on unfounded and false claims by people with clearly questionable motives."

Trump's personal lawyer, Rudolph Giuliani, had pushed for Yovanovitch's removal as he pressed Ukraine to investigate baseless corruption allegations against former vice president Joe Biden and his family.

Taylor, who had served as ambassador to Ukraine from 2006-2009, had retired from a long career in the State Department and was serving as executive vice president at the U.S. Institute of Peace when he was recruited to work in Ukraine in June after Yovanovitch's exit.

"He will lead our team during this period of historic elections and transition," the U.S. embassy in Ukraine said of Taylor after he was appointed.

A graduate of West Point and a Vietnam War veteran, Taylor has a reputation among his colleagues as a capable, reserved foreign service officer who cares deeply about supporting Ukraine's effort to counter Russian pressure.

Diplomats called to testify can draw on a legal defense fund from the American Foreign Service Association, which acts as a union for civil servants at the State Department.