Sen. Lindsey Graham wants access to phone transcripts between Vice President Joe Biden and Ukraine's leader in the month before a top prosecutor, who was alleged to have investigated the owner of the natural gas company that employed Biden's son, was fired.

President Trump and his allies have accused the elder Biden of pushing Ukraine to fire then-Prosecutor General Viktor Shokin to stop him from investigating Burisma Holdings' owner, Ukrainian oligarch Mykola Zlochevsky, in an effort to protect his son Hunter Biden.

Trump's efforts to pressure Ukraine to investigate the Bidens have become a central tenet of an impeachment investigation.

In a letter he plans to send to Secretary of State Mike Pompeo on Thursday, Graham said he will request any records of conversations Biden had with then-Ukrainian President Petro Poroshenko in February 2016.

"I want to know if there’s any transcript or readouts of the phone calls between the vice president and the president of the Ukraine in February after the raid on the gas company president’s house," the South Carolina Republican told Fox News host Sean Hannity on Wednesday.

Shokin was appointed prosecutor general of Ukraine on Feb. 10, 2015, and had taken up the investigation into Zlochevsky, but was accused of slow-walking the effort. Little more than a year later, on March 29, he was removed by Ukraine’s Parliament at Poroshenko’s behest.

Shokin's fate is now entwined with the House Democrats' impeachment proceedings.

During a July 25 phone call, which led to a whistleblower complaint that spurred the impeachment, Trump asked new Ukrainian President Volodymyr Zelensky "to do us a favor" by looking into a CrowdStrike conspiracy theory and urged him to investigate “the other thing,” referring to allegations of corruption related to the Bidens. This request stemmed from Hunter Biden’s lucrative position on the board of Burisma Holdings and the elder Biden threatening to withhold $1 billion in loans to Ukraine while pushing for Poroshenko to fire Shokin.

The Biden camp and others have countered that Shokin was seen by the United States, Europe, the International Monetary Fund, and inside Ukraine as ineffective, corrupt, and a hindrance to Ukraine’s progress. Biden was repeating U.S. policy laid out by Washington’s ambassador to Kyiv in the preceding months and was briefed by White House staff just ahead of a 2015 Ukraine trip. Ukraine’s Parliament removed Shokin in 2016, and Biden bragged on video about helping make that happen to the Council of Foreign Relations in 2018.

Impeachment witnesses have since testified that Trump's personal lawyer, Rudy Giuliani, has been involved in "shadow diplomacy" while pushing Ukrainian officials to look into the allegations against the Bidens and other Democrats. Trump has rejected accusations that he leveraged nearly $400 million in security aid and a White House meeting with Zelensky to coerce Ukraine into investigating his political rivals. Trump claims he merely wanted to root out corruption in Ukraine and has released the transcripts of two phone calls he had with Zelensky.

Having taken a leading role in the Obama administration's dealings with Ukraine, Biden made at least six trips to the Eastern European country in eight years, including in 2009, 2014, and late 2015. The last trip took place in January 2017. At the time, White House officials said the trip was to reinforce U.S. backing for Ukraine while the world prepared for the inauguration of Trump, who was expected to bring a more pro-Russia stance to office days later.

Eric Ciaramella, named as the alleged Ukraine whistleblower by RealClearInvestigations, has professional ties to Biden. He was a guest of Biden at a glitzy lunch in October 2016 to honor the prime minister of Italy. Ciaramella, 33, is listed among dozens invited by the White House Executive Office of the Vice President. On Oct. 6, the Washington Examiner reported the whistleblower worked with Biden during the Obama administration.

Ciaramella, a career CIA analyst, was Ukraine director on the National Security Council during the end of the Obama administration and remained there during the early months of the Trump administration, when he was briefly acting senior director for European and Russian affairs. The Washington Examiner reported that Ciaramella is now a deputy national intelligence officer for Russia and Eurasia on the National Intelligence Council, reporting to the director of national intelligence.

UPDATE: See Sen. Lindsey Graham's letter to Secretary of State Mike Pompeo below: