Sen. Ron Johnson, R-Wis., is preparing to subpoena a witness tied to Ukrainian natural gas company Burisma, in an escalation of the GOP probe of the firm that comes as former vice president Joe Biden’s fortunes are rising in the 2020 Democratic presidential primary.

Johnson, the chairman of the Senate Homeland Security and Governmental Affairs Committee, sent a letter Sunday to members of the panel informing them of his plan to force a vote on subpoenaing the witness, political consultant and former Ukrainian diplomat Andrii Telizhenko, who worked for a company called Blue Star Strategies that was a U.S. representative for Burisma.

Hunter Biden, Joe Biden’s son, sat on the board of Burisma while his father was the Obama administration’s point person on Ukraine policy. Trump and Republicans have claimed, without evidence, that there was something nefarious in the Bidens’ dealings with Ukraine.

If approved, Johnson’s move would mark the first subpoena Senate Republicans have issued in their probe into Biden and Burisma.

Two other GOP senators – Sens. Charles Grassley of Iowa and Lindsey Graham of South Carolina – have also launched investigations into Hunter Biden.

In his letter to members of the panel, Johnson wrote that he is “convinced obtaining Mr. Telizhenko’s Blue Star documents and information is an important part of this investigation.”

“I sincerely hope the members of the Committee will agree and support this subpoena,” he wrote.

News of Johnson’s plans was first reported by CBS News.

Johnson sent the letter one day after Joe Biden was announced the winner of the South Carolina Democratic primary, a victory that has resurrected his campaign and shaken up the race for the 2020 nomination.

Telizhenko is a 29-year-old Ukrainian national who has fueled the widely debunked theory embraced by President Donald Trump that Ukraine assisted former secretary of state Hillary Clinton in the 2016 election with help from the Democratic National Committee. The DNC has denied those claims.

Telizhenko also had a meeting last year with Trump’s personal attorney, Rudy Giuliani, who later said the two discussed U.S.-Ukraine relations.

According to a Homeland Security committee aide, Sen. Gary Peters of Michigan, the top Democrat on the panel, objected to subpoenaing Telizhenko, meaning that a committee vote is required for the subpoena to be issued.

The House impeached Trump last year on two charges – abuse of power and obstruction of Congress. The crux of the case was the allegation that Trump withheld military aid and a White House meeting to pressure Ukraine to investigate Joe Biden and his son.

The Republican-led Senate voted last month to acquit Trump on the two articles of impeachment.

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The Washington Post’s Elise Viebeck contributed to this report.