Former vice president Joe Biden lashed out Friday at Sen. Lindsey O. Graham, a longtime friend and once-close Republican ally, as Graham stepped up efforts with other GOP senators to make Biden’s son Hunter a focus of the impeachment proceedings.

The eruption shows how bitter the impeachment battle could become — personally enmeshing a top Democratic presidential candidate — as it shifts from a process controlled by House Democrats to one in which Senate Republicans attempt to reorient the focus toward Biden’s family.

Graham (R-S.C.) requested new documents Thursday from the State Department, attempting to uncover additional information related to Hunter Biden’s activities when he was on the board of the Ukrainian energy company Burisma.

“Lindsey is about to go down in a way that I think he’s going to regret his whole life,” Joe Biden said on CNN. Asked by host Don Lemon what he would say to his longtime Senate colleague, Biden responded, “I say: ‘Lindsey, I just — I’m just embarrassed by what you’re doing, for you. I mean, my Lord.’ ”

At the same time, two Republican Senate committee chairmen are seeking documents from the Treasury Department for any evidence of money laundering or fraud related to Hunter Biden’s business payments.

The increasingly personal and angry nature of the impeachment proceedings threatens to undercut a key message of Joe Biden’s campaign — that comity and civility can return to Washington after President Trump’s departure and that he’s the man to make that happen.

At the Democratic debate Wednesday night, Biden largely refrained from criticizing Republicans, even when moderators asked why he thought he could work with them given that several longtime GOP friends are calling for investigations of his family.

Biden’s reaction Thursday also reflects how deeply Washington has changed in the brief time since he left public office — with Graham now aligning himself ever more closely with Trump and Biden still insisting that Republicans can leave the Trump era behind.

“I am disappointed, and quite frankly I’m angered, by the fact — he knows me, he knows my son. He knows there’s nothing to this,” Biden said. “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.”

A Graham spokesman declined to comment and said the senator was unavailable.

Trump’s personal attorney Rudolph W. Giuliani, who has been at the center of the House impeachment inquiry, claimed that Biden’s comment was a “threat” against Graham.

“This is getting to be more and more like my old mafia cases,” Giuliani wrote on Twitter, alluding to his time as a federal prosecutor. “They sure do sound like crooks.”

Graham, in a letter sent Thursday to Secretary of State Mike Pompeo, asked for information related to calls between Biden, when he was vice president, and then-President Petro Poroshenko of Ukraine, as well as documents that referred to an investigation of Burisma.

In the Obama years, Biden played an integral role in pushing Poroshenko to crack down on corruption in Ukraine, pressuring him to fire a prosecutor, Viktor Shokin, who was widely seen as corrupt and not doing enough to undertake crucial investigations.

The efforts to oust Shokin were mounted in coordination with U.S. allies, and several Republican senators were on board at the time. Now, however, some Republicans are asserting that Biden was attempting to get rid of Shokin to protect his son, an assertion contradicted by the circumstances at the time.

Still, Biden’s aides at the time expressed concern about Hunter’s position on the board of Burisma, worried that it could create the perception of a conflict of interest. Biden took no action to discourage his son from remaining on the board.

One area of interest for Republicans is Hunter Biden’s interactions with Tony Blinken, a longtime aide to Joe Biden and a family friend who at the time was deputy secretary of state.

Hunter Biden had requested to meet with Blinken in May 2015, but the meeting never occurred because Beau Biden died later that month, according to a person familiar with the meeting. They later met in July and “they spoke about Beau and the Biden family” and they did not discuss Burisma, according to the person, who spoke on the condition of anonymity because of the sensitivity of the topic.

Graham, who ran against Trump for the Republican nomination in 2016, has transformed from one of Trump’s sharpest Republican critics to one of his fiercest defenders. Facing reelection in South Carolina, a state that voted overwhelmingly for Trump, Graham has been running interference for the president in Congress.

This week’s document request took his efforts to a new level. Graham, who chairs the Senate Judiciary Committee, told The Washington Post in late October that while he was under intense pressure to investigate Biden, he did not want to “turn the Senate into a circus” and would focus his committee’s efforts elsewhere.

Meanwhile, two of Graham’s Republican colleagues have sent a letter to a top Treasury Department official seeking any “suspicious activity reports” related to Hunter Biden, several of his business associates or companies he was affiliated with. The letter, first reported Friday by Reuters, was sent Nov. 15 by Sen. Charles E. Grassley (R-Iowa), chairman of the Senate Finance Committee, and Sen. Ron Johnson (R-Wis.), chairman of the Homeland Security and Governmental Affairs Committee.

An attorney for Hunter Biden did not immediately respond to a request for comment.

Joe Biden’s campaign noted Friday that Republicans had never previously raised questions or concerns about Hunter Biden’s activities, doing so only when Trump began facing an impeachment threat.

“Republicans controlled both chambers of Congress for most of the last five years and said or did nothing to indicate they thought that this warranted attention,” said Andrew Bates, a Biden campaign spokesman. “As recently as a few weeks ago, Lindsey Graham said that such an investigation would be a circus. What changed?”

He added, “Lindsey Graham has forfeited his conscience to escape a primary.”

Even at a time of deep partisanship, the rupture between Biden and Graham is notable — both were once known as unusually bipartisan senators willing to work across the aisle.

In the final days before Barack Obama was sworn into office as president, Biden and Graham traveled together to Afghanistan, to show that the United States had a bipartisan, unified foreign policy in the aftermath of a bitter election.

In an interview with the Huffington Post in 2015, Graham spoke emotionally about his friendship with Biden.

“If you can’t admire Joe Biden as a person, then you’ve got a problem,” Graham said. “You need to do some self-evaluation, because what’s not to like?”

He was on the verge of tears as he recounted speaking to Biden about the recent death of his son Beau.

“He’s the nicest person I think I’ve ever met in politics. He is as good a man as God ever created. I think he’s been dealt a really gut blow,” Graham said. “I think he focuses on what he’s got to do, not what he lost. His heart’s been ripped out, but he’s going to make sure that the other members of his family are well taken care of.”

As part of a series of tributes just before Biden’s term as vice president ended, Graham lavished praised on Biden, called him “a fierce competitor” who “never takes the fight too far.”

“In a political world getting more contentious by the day, with even greater divisions and an increasing lack of civility,” Graham said, “Joe Biden has always stood out.”