A Senate committee is eyeing subpoenas for current and former advisers to Joe Biden as part of an investigation into the former vice president's son, an escalation of GOP scrutiny of the presumptive Democratic presidential nominee and his family.

The Senate Homeland Security and Governmental Affairs Committee, chaired by Sen. Ron Johnson (R-Wis.), is still working to secure witness depositions voluntarily, but the negotiations have faltered in recent weeks, according to people familiar with the matter, POLITICO wrote.

Johnson is seeking testimony from former Deputy Secretary of State Antony Blinken, a senior foreign policy adviser on Biden's campaign; former special envoy for International Energy Amos Hochstein; and former senior State Department officials Victoria Nuland and Catherine Novelli.

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The panel has also scheduled an interview with David Wade, the former chief of staff to Secretary of State John Kerry. But the committee views testimony from Blinken and Hochstein in particular as critical for its forthcoming report on allegations surrounding Hunter Biden's role on the board of Burisma, a Ukrainian energy company.

The subpoenas could be authorized as soon as Wednesday, when the committee holds its next business meeting. The current agenda does not list actions related to the Burisma investigation, though that could change.

President Donald Trump has long urged his Republican allies on Capitol Hill to target his political enemies, and issuing the subpoenas would mark a key step in the probe. The potential move also comes as the president finds himself behind in most national polls and as Republicans are in danger of losing their Senate majority.

Among the subjects Johnson wants to discuss is one that first appeared in an article by conservative opinion columnist John Solomon: a memorandum of understanding signed in 2014 between Burisma and the U.S. Agency for International Development, though it does not mention either the former vice president or his son. Solomon's work at The Hill was previously faulted in an internal review following complaints about the credibility of his Ukrainian sources.

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Austin Altenburg, a spokesman for Johnson, said the committee is "not commenting on our ongoing discussions with witnesses." A spokesman for the Biden campaign declined to comment but has previously described the Johnson probe as "a political errand for Donald Trump" and an attempt "to resurrect a craven, previously debunked smear against Vice President Biden."

The committee's chief counsel previously wrote in a letter to the officials that the investigation centers on "whether certain officials within the Obama administration had actual or apparent conflicts of interest, or whether there was any other wrongdoing" associated with Hunter Biden's position as a Burisma board member.

Johnson renewed his demand for transcribed interviews and documents from the former officials days after Ukrainian lawmaker Andriy Derkach, who has met with Trump's personal lawyer Rudy Giuliani to discuss investigating the Bidens, accused Biden, his son and Hochstein in a press conference of an elaborate conspiracy to steal hundreds of millions of dollars from Ukraine.