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.

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.

Ahead of the Senate’s two-week recess earlier this month, Johnson said in a brief interview that the report was being written and finalized by members of his staff, and that he expects it to be released sometime over the summer. At the same time, Johnson has said he does not want the probe to devolve into a political spectacle.

“We’ve got the timeline laid out, we’re just trying to fill in the gaps,” Johnson said.

Johnson renewed his demand for transcribed interviews and documents from the former officials days after Ukrainian lawmaker Andrey 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.

Many of the misconduct accusations against Joe Biden, which were examined during Trump’s impeachment trial last year, have been debunked; others remain unsubstantiated.

The House impeached Trump last year on charges that he withheld critical military assistance from Ukraine in order to pressure the country’s president to announce investigations targeting Biden. The Senate acquitted Trump, and he has since sought retribution against his critics.

The committee has authorized only one subpoena so far as part of the Burisma investigation — to Blue Star Strategies, a Democratic public-affairs firm that did consulting work for Burisma. Johnson has suggested that Blue Star sought to leverage Hunter Biden’s role on the board of Burisma in order to influence matters at the Obama State Department.

The committee is also conducting a separate review of the Obama administration’s handling of the investigation into Michael Flynn during the presidential transition period in 2016 and 2017, when the incoming Trump national security adviser was scrutinized for his conversations with the Russian ambassador. Johnson has said he plans to release a report on that probe, too, later in the summer or in early autumn.

Democrats have said the twin investigations are intended simply to boost Trump’s reelection prospects. They also contend the probes are a misuse of the Senate’s resources and are likely contributing to Russian disinformation campaigns.

On Thursday, Sen. Gary Peters of Michigan, the top Democrat on the Homeland Security Committee, sent a letter to Johnson re-upping his demand for a briefing from the FBI’s Foreign Influence Task Force and intelligence community officials.

Peters and Oregon Sen. Ron Wyden, the top Democrat on the Senate Finance Committee, said the briefings are necessary in order to “fully understand the national security and counterintelligence implications of foreign election interference and your ongoing investigative work.” The letter was also addressed to Senate Finance Chairman Chuck Grassley (R-Iowa), who has joined Johnson in many of his investigations.

Trump has openly encouraged both investigations as he uses them as a key theme of his bid for a second term — that he was unfairly targeted by the Obama White House with a slew of federal probes into Russian interference in the 2016 election and claims that the Trump campaign conspired with Russian operatives.