ROCHESTER, N.H. — Former Vice President Joe Biden made his most direct call yet for President Trump’s impeachment Wednesday, just hours after Trump tweeted that the Democratic-led inquiry was tainted with political bias and should be terminated “for the good of the Country.”

China, Biden said, “was the third foreign power that we know of that [Trump] has asked in clear, unmistakable language to intervene on his behalf in the democratic proceedings of the United States,” the former vice president said, referring to public remarks Trump made last week on the White House lawn.

“President Trump has indicted himself by obstructing justice, refusing to comply with a congressional inquiry … he’s already convicted himself,” Biden said.

“In full view of the American people, Donald Trump has violated his oath of office, betrayed this nation and committed impeachable acts. To preserve our Constitution, our democracy and our basic integrity, he should be impeached,” he said..

The comments from the Democratic presidential hopeful came a day after the White House said in a scathing eight-page letter that it would not cooperate with the inquiry into the Ukraine scandal on the grounds that it lacked merit.

The letter was the latest escalation in a standoff with Congress, where Democrats are vowing to hold Trump accountable for pressing Ukrainian President Volodymyr Zelensky this year to investigate Biden and his son, Hunter Biden, at a time when U.S. military aid to Ukraine had been suspended.

Joe Biden told a crowd of about a hundred people in New Hampshire that the move was an effort by Trump to “smear” him in order to pick his own opponent in 2020. The former vice president said that neither he nor his son did anything illegal, but he did not offer a detailed defense of Hunter Biden’s actions in Ukraine.

Before Joe Biden left the venue where his speech was held, Trump took to Twitter to respond.

“So pathetic to see Sleepy Joe Biden, who with his son, Hunter, and to the detriment of the American Taxpayer, has ripped off at least two countries for millions of dollars, calling for my impeachment – and I did nothing wrong,” Trump tweeted. “Joe’s Failing Campaign gave him no other choice!”

Biden replied in kind: “Thanks for watching,” he tweeted. “Stop stonewalling the Congress. Honor your oath. Respect the Constitution. And speaking of taxpayers, I’ve released 21 years of my tax returns. You?”

Hunter Biden served for nearly five years on the board of Burisma, Ukraine’s largest private gas company, whose owner had earlier come under scrutiny by Ukrainian prosecutors over possible abuse of power and unlawful enrichment. The younger Biden was not accused of any wrongdoing in the investigation. The investigation into Burisma was dormant at the time that Joe Biden, acting as the Obama administration’s point person on Ukraine, sought to have Viktor Shokin, the Ukrainian prosecutor, ousted, according to former Ukrainian and U.S. officials.

Biden reminded his audience Wednesday that other Western officials and even some Republican lawmakers agreed that Shokin needed to be replaced because he was not sufficiently pursuing corruption cases in Ukraine.

On the other hand, Biden said, Trump has repeatedly called on foreign powers – including China, during televised remarks to reporters last week on the White House lawn – to investigate the president’s political enemies. Those statements were similar to remarks Trump made in 2016 when he suggested that the Russian government should steal his Democratic opponent Hillary Clinton’s emails and release them.

On the campaign trail, Biden has sought to turn the conversation away from Ukraine and instead focus on education and the middle class, even as impeachment has dominated headlines.

Still, many Democrats, including Biden partisans, have begun to worry that Trump’s relentless attacks on the former vice president will weigh on the campaign, even if they’re baseless. If Biden is the Democratic nominee, there is fear that Trump could succeed in sowing doubt in voters’ minds.

Just hours after Biden spoke, the president’s reelection campaign announced a multi-ad buy – totaling $10 million and focused in early-voting states – that focuses on the Bidens and Ukraine. The ad campaign repeats Trump’s unfounded accusation that Biden had leaned on Ukraine to fire its top prosecutor to benefit his son. It also casts the impeachment hearings as a Democratic attempt to overturn the 2016 election.