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 representative for Burisma in the United States.

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. President Trump and Republicans have claimed, without evidence, that there was something nefarious in the Bidens’ dealings with Ukraine.

Trump and his allies have focused primarily on Biden’s push to have Ukraine’s top prosecutor, Viktor Shokin, removed — a position supported at the time by other Western leaders who said Shokin was not doing enough to root out corruption in Ukraine. The prosecutor had previously investigated Burisma, but the probe was dormant at the time, according to former Ukrainian and U.S. officials.

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 — Charles E. Grassley (Iowa) and Lindsey O. Graham (S.C.) — 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.

In an interview Monday night, Johnson denied that Biden’s win sparked his subpoena request, noting that “we’d still be asking for the same information” regardless of Biden’s status as a Democratic presidential candidate.

“My oversight is not directed toward Joe Biden or Hunter Biden,” Johnson said. “It’s these issues — it just so happens that Joe Biden and Hunter Biden are also wrapped up and connected to the whole Ukrainian issue.”

Last Monday, Johnson sent a letter to Sen. Gary Peters (Mich.), the top Democrat on the panel, advising him of his intent to issue the subpoena. Three days later, Peters wrote back to Johnson voicing his objections to subpoenaing Telizhenko and calling for the committee to vote on the matter “given the significant national security concerns” involved, particularly on the issue of Russian disinformation efforts.

Telizhenko is a 29-year-old Ukrainian national who has fueled the widely debunked theory embraced by 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, Rudolph W. Giuliani, who later said the two discussed U.S.-Ukraine relations.

In a statement Monday, Peters reiterated his concern that the committee could be used to further Russian disinformation efforts.

“Our national security and intelligence experts, and our own Senate Intelligence Committee, have clearly shown that there is a concerted Russian effort to interfere in our 2020 elections,” Peters said. “We need to take every step to ensure the credibility and resources of the U.S. Senate are not used to advance interference efforts by foreign adversaries that seek to undermine our democracy or put our national security at risk.”

Johnson argued that the news of the planned subpoena would not have been made public on Monday had Peters not objected to his proposal.

“The timing here — you wouldn’t even know about this had Sen. Peters not objected,” Johnson said. “I urged him, ‘Don’t object, we’ll just issue the subpoena quietly, we’ll get the documents, we’ll continue our oversight work.’ ”

Asked whether he believes he has the votes to issue the subpoena, Johnson replied, “I don’t know why anybody would object to this.”

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.

Elise Viebeck and Colby Itkowitz contributed to this report.