 -- One of President Donald Trump’s closest confidants, his personal lawyer Michael Cohen, has now become a focus of the expanding congressional investigation into Russian efforts to influence the 2016 campaign.

Cohen confirmed to ABC News that House and Senate investigators have asked him “to provide information and testimony” about any contacts he had with people connected to the Russian government, but he said he has turned down the invitation.

“I declined the invitation to participate, as the request was poorly phrased, overly broad and not capable of being answered,” Cohen told ABC News in an email Tuesday.

After Cohen rejected the congressional requests for cooperation, the Senate Select Intelligence Committee voted unanimously on Thursday to grant its chairman, Sen. Richard Burr of North Carolina, and ranking Democrat, Sen. Mark Warner of Virginia, blanket authority to issue subpoenas as they deem necessary.

"To date, there has not been a single witness, document or piece of evidence linking me to this fake Russian conspiracy," Cohen added. "This is not surprising to me because there is none."

While much of the media focus in recent days has fallen on Russian contacts made by Trump’s son-in-law, Jared Kushner, there are few people closer to the president than his longtime lawyer. Insiders consider Cohen to be Trump’s pit bull or consigliere for his role in threatening legal action against Trump critics, gaining notoriety for threatening and browbeating reporters investigating Trump’s background.

He was quoted in 2015 telling Daily Beast reporters, “I will take you for every penny you still don’t have. And I will come after your Daily Beast and everybody else that you possibly know … So I’m warning you, tread very f---ing lightly, because what I’m going to do to you is going to be f---ing disgusting.”

In a 2016 appearance on CNN that went viral, the stone-faced attorney flashed anger when anchor Brianna Keiler said the Trump campaign was “down.”

“Says who?” he challenged. When she cited polls, he countered, “Which polls?” She replied, “All of them.” His final response in that exchange proved prescient: “You’re going to all be very surprised when he polls substantially higher than what you all are giving him credit for.”

After the 2016 campaign, Cohen left the Trump Organization to become the president’s personal attorney, a job he still holds. From that post, he has continued to weigh in on Trump’s behalf on Twitter and during occasional television appearances.

After Trump dismissed FBI Director James Comey, for example, Cohen tweeted, “I believe @POTUS was justified in terminating #Comey as @FBI director. #RT if you agree with me!”

Cohen was also made a deputy national finance chairman of the Republican National Committee — a position that gives him some sway on how money will be allocated to Republican candidates. And in April he announced he formed a “strategic alliance” with the powerful D.C. lobbying firm Patton Boggs, a firm whose clients include Russia’s third-largest bank, Gazprombank. The arrangement enables him to work out of Squire Patton Boggs’ offices in New York, Washington and London, according to the announcement.

The emergence of Cohen as a subject of the Senate probe brings renewed attention to a strident Trump advocate who was named in the unverified dossier prepared by a former British intelligence agent during the 2016 campaign and provided by the FBI to Sen. John McCain, which contains a number of unconfirmed allegations that Cohen played a role in working with the Russians on the hacking of Democratic National Committee computers during the campaign.

In January, Cohen told ABC News the allegations in the dossier were “laughably false.” His wife is Ukrainian, and he once worked with her family in Ukraine to establish an ethanol business. ABC News was able to debunk some references to him in the unverified document, such as the assertion in the that his Ukrainian-born father-in-law had a vacation home, or dacha, near Russian President Vladimir Putin’s.

“I don’t even think my father-in-law has ever been to Moscow,” Cohen told ABC News earlier this year. “I wonder who’s living in the dacha.”

Another suggestion in those documents — that Cohen supposedly met with the Russians in Prague last August — is also false, he said. Then-President-elect Trump pushed back against the claim in a wide-ranging news conference held in January, saying that he saw Cohen’s passport.

“I said, ‘I want to see your passport.’ He brings his passport to my office. I say, ‘Hey, wait a minute. He didn’t leave the country. He wasn’t out of the country.’ They had Michael Cohen of the Trump Organization was in Prague. It turned out to be a different Michael Cohen,” Trump said. “It’s a disgrace what took place. It’s a disgrace, and I think they ought to apologize to start with Michael Cohen.”

Democrats in Congress have argued it is conceivable he entered Europe through another country — he was in Italy on vacation around the time the dossier alleges he was in Prague — and his passport would not receive a stamp for crossing the border, but no proof of any such trip has been produced.

“I’ve never actually walked the land in Prague,” Cohen told ABC News. “And last August I was not in Prague.”

Congressional investigators involved in the widening probe have already identified four Trump campaign advisers as people of interest because of their interactions with Russian officials. Only one of them, retired Lt. Gen. Michael Flynn, has received a subpoena for records. Flynn, who served briefly as Trump’s national security adviser, declined to provide them, citing his Fifth Amendment rights.

Lawmakers have also asked former Trump campaign chairman Paul Manafort, informal adviser Roger Stone and former foreign policy adviser Carter Page to voluntarily hand over relevant records. All three men have said publicly they are producing records and cooperating with investigators.

ABC News' Eric Avram and Pete Madden contributed to this report.