Sen. Lindsey Graham (R-SC) on Monday defended his decision to investigate former Vice President Joe Biden and his son, Hunter Biden, over allegations of corruption related to their activities in Ukraine.

Graham sent a letter to Secretary of State Mike Pompeo on Thursday seeking documents in an effort to determine whether the former vice president was involved for the firing of Ukrainian Prosecutor General Viktor Shokin in a bid to squash a probe into Burisma Holdings, the Ukrainian gas firm where the younger was a board member. Hunter Biden was paid up to $83,000 a month for several years as a board member, while his father was in the White House.

“My conscience is clear. I love Joe Biden as a person, he is a really decent man, he’s had a lot of tragedy in his life, but I have a conscience very clear right now. And I have a duty, if the House is going to shut it down the Senate is going to pick it up,” Graham told reporters in Charleston, South Carolina, according to The Hill.

“I’m not saying Joe did anything wrong, but I want to see the transcripts, and if there’s nothing there I’ll be the first one to say there’s nothing there,” he added. “I believe that Hunter Biden’s association on that board doesn’t pass the smell test. If a Republican was in the same boat they would be eaten alive by the media.”

Biden, a top-tier presidential candidate, reacted angrily to the probe, warning Graham will “regret” the move his “whole life.”

“Lindsey is about to go down in a way that I think he’s going to regret his whole life,” Biden told CNN host Don Lemon in an interview Friday. “I’m just embarrassed by what you’re doing, for you. I mean, my Lord.”

“They have [Graham] under their thumb right now. They know he knows that if he comes out against Trump, he’s got a real tough road for reelection, number one,” he continued.

“He knows me; he knows my son; he knows there’s nothing to this,” Biden then vented. “Trump is now essentially holding power over him that even the Ukrainians wouldn’t yield to. The Ukrainians would not yield to, quote, ‘investigate Biden’ — there’s nothing to investigate about Biden or his son.”

Graham has also requested all documents and communications related to Biden’s phone calls with Poroshenko on February 11, 18 and 19, and March 22, 2016, citing media reports that they discussed previous demands to dismiss Shokin for alleged corruption before he was removed from office on March 29, 2016.

Further, the senator is seeking all documents and communications related to a meeting between Devon Archer, a business partner of Hunter Biden, and then-Secretary of State John Kerry on March 2, 2016.

His request comes after two weeks of public testimony on the impeachment inquiry into President Donald Trump, which centers around a “whistleblower” complaint that the president withheld military aid from Ukraine in an attempt to pressure newly elected Ukrainian President Volodymyr Zelensky to investigate Burisma and the Bidens.

The UPI contributed to this report.