Joe Biden ruled out the possibility that he would voluntarily testify in the impeachment proceedings against President Donald Trump on Wednesday, saying to do so would “divert” away from the issue at hand.

“No, I’m not going to let them take their eye off the ball,” the former vice president told reporters when asked about the possibility on his “No Malarkey” bus tour in Iowa. “The president is the one who has committed impeachable crimes, and I’m not going to let him divert from that. I’m not going to let anyone divert from that.”

Biden’s refusal sets up the likelihood that he would need to be subpoenaed to appear before Congress, especially if the impeachment inquiry proceeds to a trial in the Senate. Some Republicans, like Sens. Lindsey Graham (R-SC) and Ron Johnson (R-WI), have already signaled it would be inappropriate for the impeachment inquiry, let alone a trial, to progress without the testimonies of both the former vice president and his youngest son, Hunter. As such, Senate Republicans have begun looking into the Obama-era White House and Hunter Biden’s wheeling and dealing in Ukraine, which has taken center stage in the inquiry.

The former vice president, for his part, has responded to calls that he and his son testify by lashing out at the Republicans, like Graham.

“Lindsey is about to go down in a way that I think he’s going to regret his whole life,” Biden said last month when asked about the senator’s efforts around impeachment. “I say Lindsey, I just—I’m just embarrassed by what you’re doing, for you. I mean, my Lord.”

Biden’s refusal to testify also underscores just how central he and his son are to the Democrats’ case for impeaching Trump. The controversy started when Trump suggested the Ukrainian government investigate how Hunter Biden was able to secure a seat on the board of directors of Burisma Holdings. The younger Biden was appointed to the Ukraine-based natural gas company’s board in 2014, despite having no background in either the energy industry or eastern Europe. More troubling was the fact that Hunter Biden’s appointment seemed to coincide with his father being tapped to lead the Obama administration’s policy towards in Ukraine in response to Russia’s invasion of Crimea.

As Peter Schweizer, senior contributor at Breitbart News, detailed in his book Secret Empires: How the American Political Class Hides Corruption and Enriches Family and Friends, Hunter Biden’s background in investment banking, lobbying, and hedge fund management paled in comparison to that of current and past members of Burisma’s board.

Adding to concerns is the fact that at the time Hunter Biden joined Burisma, the company was seen as actively courting leaders in the West to prevent further scrutiny of its business practices. The same month that Hunter Biden was tapped to join the company’s board, the government of Great Britain froze accounts belonging to Burisma’s founder, Mykola Zlochevsky, under suspicion of money laundering.

A Ukrainian official with strong ties to Zlochevsky admitted in October the only reason that Hunter Biden secured the appointment was to “protect” the company from foreign scrutiny.

It is in the context of Burisma and Zlochevsky’s legal troubles that Joe Biden’s political influence has raised the most red flags. The former vice president has particularly drawn questions over his conduct in demanding the Ukrainian government fire its top prosecutor, Viktor Shokin, in 2016.

Joe Biden, who has publicly bragged about the firing, reportedly threatened to withhold more than one billion dollars in U.S. aid if the Ukrainian government did not remove Shokin. He has claimed the demand came from then-President Barack Obama, who had allegedly lost faith in the prosecutor’s ability to tackle corruption.

Unofficially, though, it was known that Shokin was investigating both Burisma and Zlochevsky for public corruption. It is uncertain if the probe extended to Hunter Biden, although Shokin has recently admitted that prior to his ouster, he was warned to back off the matter. Regardless of what occurred, Shokin’s successor, who is now himself being investigated for public corruption, dropped the investigation into Burisma.

Congressional Republicans have cited the shadowy timeline of events and the appearance of conflicting interests when arguing for Hunter and Joe Biden to testify before the impeachment proceedings.

“I believe that Hunter Biden’s association on that board doesn’t pass the smell test,” Graham told reporters last month. “If a Republican was in the same boat they would be eaten alive by the media.”