Washington (CNN) Senate Judiciary Committee Chairman Lindsey Graham is seeking documents related to former Vice President Joe Biden and his son Hunter Biden's previous work in Ukraine, as President Donald Trump and his Republican allies have called for an investigation into the Democratic 2020 hopeful .

In a letter dated Thursday, Graham, a Republican from South Carolina, wrote to Secretary of State Mike Pompeo, requesting documents from the State Department of Joe Biden's phone calls with former Ukrainian President Petro Poroshenko in February and March 2016 -- including whether there was any mention of the country's investigation into the business activities of Mykola Zlochevsky , who owned the natural gas company, Burisma Holdings.

President Donald Trump and his allies have pushed allegations that Joe Biden improperly pressured the Ukrainian government to fire former Ukraine Prosecutor General Viktor Shokin in efforts to stop an investigation into Burisma Holdings, which Hunter Biden sat on the board of directors.

There has been no evidence of wrongdoing by the Bidens.

Graham also requested documents related to a March 2, 2016, meeting between Devon Archer, a business partner of Hunter Biden, and then-Secretary of State John Kerry.

CNN has reached to the State Department for comment.

Graham back in September had said the Ukraine-related claims against the Bidens need to be investigated, but said he wouldn't use the Judiciary Committee he chairs to because he is close to the former vice president and his late son, Beau.

In a letter Thursday, Republican Sens. Ron Johnson of Wisconsin and Chuck Grassley of Iowa requested records from the US National Archives regarding White House meetings in 2016 between Obama officials, Ukrainian government representatives and Democratic National Committee officials.

The GOP senators cite a conservative columnist's report that during a January 2016 meeting Obama officials raised the investigations into Burisma. The deadline to hand over the documents is December 5.

The Republican senators' requests come as the House impeachment inquiry heard public testimony from current and former US officials and diplomats about an "irregular" policy channel established for Ukraine and that the Trump administration tried to use a White House invitation and US military assistance as leverage to pressure the new Ukrainian government into announcing investigation into the Bidens.

Pompeo, who was on a July call between Trump and the Ukraine President in which he asked for a "favor," has defied subpoenas from House Democrats in their impeachment inquiry into Trump, declining to turn over documents related to Trump and Ukraine.

A central figure in the impeachment inquiry, US Ambassador to the European Union, Gordon Sondland, testified before Congress that he had made multiple requests to the State Department for his department phone records, emails and other documents to help him testify, but that "these materials were not provided" to him or the committee. A State Department official said Sondland "retained at all times, and continues to retain, full access to his State Department documentary records and his State Department email account."

The department also repeatedly attempted to block its diplomats from testifying, leaving impeachment investigators to resort to subpoenas to bring in the witnesses.

It is unclear if Pompeo might take a different position for Graham's investigation, but such a move could further politicize the department and create a wider division between its political appointees and career diplomats.

Republicans on the House Intelligence Committee earlier this month included Hunter Biden and Archer among a list of witnesses they wanted to testify before the committee in the impeachment inquiry.

In an interview with ABC News , Hunter Biden admitted that it was "poor judgment" for him to sit on the board of Burisma, while his father was in charge of Ukraine policy in the Obama administration. But he insisted he did "nothing wrong at all."

Trump and his personal lawyer Rudy Giuliani have pushed this allegations of corruption about the Bidens. Trump in a July 25 phone call asked Ukrainian President Volodymyr Zelensky to investigate the Bidens, a request that came as the US was withholding military aid to Ukraine. The call has been the focus of House Democrats' impeachment inquiry.

After he became prosecutor general in 2015, Shokin was widely faulted for declining to bring prosecutions of elites' corruption. The Obama administration, American allies, the International Monetary Fund and Ukrainian anti-corruption activists, among others, called for Shokin's removal.

This story has been updated Friday with additional developments.