Washington — As Democrats and Republicans in Congress battle over impeachment, GOP Senator Lindsey Graham says he's pushing forward with his own probe of a close friend, former vice president and current 2020 contender Joe Biden.

"I love Joe Biden, but none of us are above scrutiny," Graham told "Face the Nation," when asked about Biden's recent comment that Graham's approach to the Ukraine scandal is something he would "regret his whole life." Graham has promised to look into son Hunter Biden's work for Ukrainian gas company Burisma, echoing a demand from President Trump that's at the center of the impeachment proceedings against him.

"You want to talk about Ukraine, it's pretty hard for me to go home and tell my constituents to ignore the fact that Hunter Biden received $50,000 a month from a gas company in the Ukraine, run by the most corrupt person in the Ukraine, and two months after the gas company was investigated, the prosecutor got fired," Graham told "Face the Nation."

Hunter Biden was hired to serve on the board of Burisma in 2014 when his father was vice president. In 2016, the elder Biden successfully pushed for the firing of Ukraine's prosecutor-general, who was widely seen as unwilling or unable to crack down on corruption. Mr. Trump and his allies have accused Biden of pushing for the prosecutor's ouster to protect Burisma, but the company was not under investigation at the time of the firing, and the U.S. and Western allies were unanimous in their belief that the prosecutor should be fired.

Graham, the chairman of the Senate Judiciary Committee, sent a letter last month requesting documents from the State Department, including records of calls between Biden and then-Ukrainian President Petro Poroshenko about Burisma and the fired prosecutor.

"Joe Biden is a friend. He's one of the most decent people I've ever met in my life. But here's the deal. This whole process around the Ukraine reeks with politics," Graham said. "They've done everything but take a wrecking ball to Donald Trump and his family. We're not going to live in a world where only Republicans get looked at."

While it's not clear if the State Department will comply with Graham's requests, the move was part of a full-court press by Republicans to defend Mr. Trump's dealings with Ukraine by redirecting attention to the Bidens.

"I don't know if there's anything to this. I hope not. I hope I can look at the transcripts of the phone call between Biden and the Ukraine, Joe Biden, after the investigation began, and say there's no there, there. These are legitimate concerns about what happened in the Ukraine," Graham said.

Graham also suggested the White House would be interested in calling both Bidens as witnesses as the probe moves to the Senate trial. But Graham said he is "ready to vote on the underlying articles. I don't really need to hear a lot of witnesses."