The request, made by Senator Ron Johnson (R., Wisc.) — who chairs the Homeland Security Committee — as part of a larger probe into a potential conflict of interest between Hunter Biden’s role at Burisma and Joe Biden’s diplomatic work in the country. The subpoena targets Blue Star Strategies, a Democratic consulting firm that worked on a campaign to rehabilitate Burisma’s image in Washington.

Senator Mitt Romney (R., Utah), who had initially expressed concerns on the probe, voted for Johnson’s subpoena. In March, Romney said the effort “appears political,” but eventually backed the probe after speaking with Johnson and being reassured that his subpoena would occur “without a hearing or public spectacle.”

Johnson has said that Blue Star attempted to “leverage Hunter Biden’s role as a board member of Burisma to gain access to, and potentially influence matters at, the State Department.” In March, the Wisconsin Republican said he had a document showing Blue Star apologizing for a “misinformation campaign” against Ukrainian prosecutor Victor Shokin, whom Joe Biden bragged about getting fired. Emails reported on in November show the firm mentioning Hunter Biden, who then served on Burisma’s board, in a request for company executives to meet with State Department officials.

Democrats have reacted negatively to Johnson’s efforts. Senator Gary Peters (D., Mich.), the ranking member of the committee, warned after the vote that “this extremely partisan investigation” is “going down a dangerous road.” A campaign spokesman for Joe Biden told the Washington Post that Johnson is “running a political errand for Donald Trump” by “attempting to resurrect a craven, previously-debunked smear against Vice President Biden.”

Johnson replied to the disapproval by saying “I think they protest a little bit too much.”

“Apparently we’re hitting a nerve here,” he told Fox News. “Maybe we’re getting close to finding some important information.” He has previously said the report would be published “sometime this summer.”