The timeline for a report on a GOP investigation into the Obama administration and Hunter Biden, the son of presumptive Democratic presidential nominee Joe Biden, is slipping, with a top Senate chairman predicting he will soon issue subpoenas. 

Sen. Ron Johnson (R-Wis.), the chairman of the Homeland Security and Governmental Affairs Committee, told The Hill on Wednesday that he now wants to release the interim report by mid-September, citing difficulty in setting up interviews. 

"I don't see a report coming out here before August. I think a lot of interviews are going to be conducted in August," Johnson said when asked about the timeline for the report. "We've kind of set a target date for Sept. 15. And I've said, 'OK, guys, that has to be hard and fast.'" 

The timeline is a shift from last month when Johnson said he had told staff he wanted to release the report before the August recess, which is scheduled to start on Aug. 7, "as incomplete as it is."

"At some point in time we've got [to say], 'OK, here’s what we’ve got. Here’s the remaining questions that need to be answered,'" Johnson said at the time. 

Johnson and Sen. Chuck Grassley (R-Iowa) began closed-door depositions last week, but Johnson hinted on Wednesday that he will soon be issuing subpoenas, though he declined to specify a specific timeline or who will be targeted. 

"We're trying to work with all these people to bring them in voluntarily, but even that takes time. There's wrangling over conditions on interviews. It just takes way more time," Johnson said. "You might start seeing some subpoenas too."  

Johnson and Grassley, the chairman of the Senate Finance Committee, previously sent a letter in December requesting interviews with former Obama officials including Antony Blinken, the former deputy secretary of State, and Amos Hochstein, the former senior adviser on international energy affairs to then-Vice President Joe Biden. Johnson's staff then sent follow-up requests last month. 

Asked about the status of talks with Blinken and Hochstein, Johnson added that they were in discussions but that "progress is a relative term."

"At some point in time we could probably start seeing some subpoenas being issued," Johnson said.

Johnson and Grassley are months into their investigation into Obama-era decisions. Johnson has said repeatedly that they aren't targeting Hunter Biden and that the probe isn't driven by his father running for president. 

But the two are, as part of the investigation, asking for Hunter Biden's travel records; information related to Burisma Holdings, where Hunter Biden previously served on the board; and outreach between the Obama State Department and Hunter Biden's business connections.

The probe has increasingly become a target for Democrats, who worry Johnson will inadvertently spread Russian misinformation and is using his gavel to target President Trump's political enemies. 

Johnson and Grassley, in a joint letter this week, accused Democrats of trying to undermine their investigation by leaking information. 

"We hope that you will join us in rejecting any and all efforts by foreign entities to interfere in our elections, and refrain from using this issue as a political weapon to target investigations — investigations grounded in fact and that fall squarely within the Committees’ jurisdiction to safeguard our homeland security and financial systems —with which you happen to disagree," they wrote.