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Senator Ron Johnson, Republican of Wisconsin, the chairman of the Homeland Security and Governmental Affairs Committee, was under pressure from Democrats and members of his own party after a pair of briefings from federal officials on Tuesday. The briefings raised questions about the Ukrainian's reliability as a source for two politically sensitive investigations before the committee, including one into the family of former Vice President Joseph R. Biden Jr., according to The New York Times.

Mr. Johnson told lawmakers Wednesday afternoon he would delay his pursuit of additional documents from the former Ukrainian official, Andrii Telizhenko, until intelligence officials could provide more information to lawmakers.

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In a letter to fellow lawmakers Wednesday afternoon just before the scheduled vote, Mr. Johnson told his colleagues he was postponing the vote "out of an abundance of caution, and to allow time for you to receive additional briefings."

Instead, Mr. Johnson said he would move to issue a subpoena to Blue Star Strategies, a Washington lobbying firm for which Mr. Telizhenko worked on behalf of clients, including a Ukrainian energy company that paid Mr. Biden's son, Hunter Biden, as a board member.

"There was a suggestion made by both Democrat and Republican senators: Why don't you just go straight to the source, Blue Star Strategies, which is what I decided to do," Mr. Johnson said in an interview.

Staff members for the committee have been working with Mr. Telizhenko for months, interviewing him and collecting documents to bolster two separate investigations into subjects that could help President Trump as he heads into a re-election campaign in which Mr. Biden has emerged as his leading Democratic challenger. The White House is closely monitoring Mr. Johnson's efforts on the investigations, according to a person familiar with the situation.

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One of the investigations deals with the overlap between Hunter Biden's work for Burisma Holdings, a Ukrainian energy company widely accused of corruption, and his father's diplomacy in Ukraine. The other relates to claims that Ukrainian officials tried to undermine Mr. Trump during his 2016 campaign.

Mr. Telizhenko has firsthand experience in both subjects. Before his work with Blue Star on behalf of Burisma, he worked for Ukraine's top prosecutor and then in Ukraine's Embassy in Washington, where, he claims, he was instructed by a superior to work with a Democratic operative to collect and disseminate damaging information about Mr. Trump's 2016 campaign chairman, Paul Manafort. Republicans seized on those claims to justify Mr. Trump's push for investigations and, at times, to answer concerns about Russia's interference to help Mr. Trump in 2016.

American intelligence agencies have concerns about Mr. Telizhenko's role in spreading disinformation about Ukraine's involvement in the 2016 election, according to people familiar with the matter.

Intelligence agencies have contended for months that Russian intelligence has worked to amplify, spread and distort stories relating to the involvement of Ukrainian officials in trying to influence the outcome of the last presidential election, as part of a campaign to minimize or obscure Moscow's own efforts.

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Mr. Telizhenko, 29, was raised partly in Canada, attended the University of the District of Columbia and speaks fluent English. During the 2014 uprising that ousted Ukraine's Russia-aligned president, Mr. Telizhenko helped the protesters connect with Western politicians including Senator John McCain, whose funeral he attended in 2018.

Mr. Telizhenko says he is a political conservative and has been hanging out at the Trump International Hotel in Washington, appearing on the conservative One America News Network and forging a bond with Mr. Trump's personal lawyer Rudolph W. Giuliani, whose efforts to uncover politically useful information have been assisted by Mr. Telizhenko.

Mr. Telizhenko, who has been a private consultant since leaving his role with the embassy in 2016, declined to identify his clients. But someone familiar with his work said that he had until recently represented the Ukrainian-Russian developer Pavel Fuks.