An associate of Sen. John McCain John Sidney McCainMomentum growing among Republicans for Supreme Court vote before Election Day McConnell urges GOP senators to 'keep your powder dry' on Supreme Court vacancy McSally says current Senate should vote on Trump nominee MORE (R-Ariz.) invoked the Fifth Amendment in order to not testify before the House Intelligence Committee on a dossier of opposition research that claims ties between President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE and Russia, according to Fox News.

The committee originally issued a subpoena for David Kramer — a former State Department official and current senior director of the nonprofit McCain Institute — in December.

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Committee Chairman Devin Nunes Devin Gerald NunesSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Sunday shows preview: With less than two months to go, race for the White House heats up Sunday shows preview: Republicans gear up for national convention, USPS debate continues in Washington MORE (R-Calif.) had wanted to speak with Kramer about his visit to London in November 2016, during which he met with the author of the dossier, former British intelligence officer Christopher Steele. According to court filings, he met with Steele at McCain's request, to view “the pre-election memoranda on a confidential basis."

Kramer then traveled back to the U.S. and gave copies of the memos to McCain, who in turn handed the documents over to the FBI.

The committee interviewed Kramer last month prior to issuing a subpoena for him.

The dossier was contracted out to opposition research firm Fusion GPS and funded in part by Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE's campaign.

President Trump has called the document "fake."

Republicans want to know if the document was used as the basis for a surveillance warrant on any members of the Trump campaign.

However, Democrats want to know if the salacious claims about Trump in the dossier, which connect him to Russia, are true.