WASHINGTON — A day before the opening of President Donald Trump's Senate impeachment trial, former Vice President Joe Biden's campaign is issuing a pre-emptive strike, sending an unusual open memo to the media warning against disinformation pushed by the president and his defenders.

The memo, first obtained by NBC News, is also a shot across the bow of Republican senators as they consider whether to entertain Trump's demands to call Hunter Biden, Joe Biden's son, as a witness in the trial.

Trump's attempts to withhold nearly $400 million in military aid to Ukraine while pressing it to investigate Joe Biden, a chief political rival, led Trump to become the third president to be impeached by the House of Representatives.

Among the issues addressed in the memo is the past targeting of Biden's attempt as vice president to sideline Viktor Shokin, a Russia-aligned Ukrainian prosecutor general, at a time when Biden's son served on the board of Burisma, an energy company in Ukraine.

Trump has been "spreading a malicious and conclusively debunked conspiracy theory" that "Biden engaged in wrongdoing when he executed official United States policy to remove a corrupt prosecutor from office," the campaign said in the memo sent by Biden's deputy campaign manager, Kate Bedingfield, and a senior adviser, Tony Blinken.

Current and former administration officials testified during the House impeachment investigation that while Hunter Biden's role on the board of Burisma presented the potential for the appearance of a conflict of interest given his father's position, Shokin's ouster was a key priority of the Obama administration and the international community.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

In its first forceful pushback since impeachment proceedings began, the Biden campaign is demanding that "any media organization referencing, reporting on, or repeating these claims" from Trump and his allies "must state clearly and unambiguously that they have been discredited and debunked by authoritative sources."

Download the NBC News app for breaking news and politics

The memo reflects, in part, the new political reality as proceedings go from the Democratic-led House to the Republican-led Senate, where Hunter Biden could be called as a witness.

Democrats have long insisted that any testimony Hunter Biden might give is immaterial to determining Trump's motives for withholding the aid to Ukraine. Democrats defeated multiple attempts by Republicans to seek such testimony in the House proceedings.

The Biden campaign put off creating a "war room" rapid response effort during the House proceedings, because it believed that the testimony from administration officials was a powerful enough antidote to GOP claims.

But Republicans are in the majority in the Senate, and many will "play this broken record over and over again during the Senate trial," the Biden campaign said. "It is not sufficient to say the allegations are 'unsubstantiated' or that 'no evidence has emerged to support them.' Not only is there 'no evidence' for Republicans' main argument against the Vice President — there is a mountain of evidence that actively debunks it. And it is malpractice to ignore that truth."

Among the many fact checks the campaign is highlighting anew are those from The Associated Press and The Washington Post showing that the investigation into Burisma was dormant by the time Biden sought Shokin's ouster.

Following Biden's April campaign announcement, Trump's inner circle, led by personal lawyer Rudy Giuliani, intensified efforts to pressure Ukraine to announce an investigation of the Bidens. Trump approved the same aid in 2018 with no such conditions. The Defense Department and Congress had cleared it, and Republicans, including Sen. Rob Portman of Ohio, were hailing Ukraine's new president as a reformer.

Shokin's ouster was supported by Portman; Sen. Ron Johnson, R-Wis.; the European Union; and global financial institutions, including the International Monetary Fund. Ukraine's parliament overwhelmingly voted him out.

Furthermore, Hunter Biden joined the board two years after the "corruption" at issue took place. It revolves around Burisma's owner, Mykola Zlochevsky, a multimillionaire former minister of ecology and natural resources, as well as tax violations, money-laundering and licenses given to Burisma.

In a series of interviews in May and October, the prosecutor who replaced Shokin, Yuriy Lutsenko, said he could find no evidence of wrongdoing involving the Bidens and violations of Ukrainian law, although he told NBC News that the situation did have the appearance of a "conflict of interest" for the Bidens.

Daria Kaleniuk, director of the Anti-Corruption Action Center in Ukraine, said it was "unethical" for Hunter Biden to trade on his father's governmental position by serving on Burisma's board. But Joe Biden "did the opposite" of helping Burisma by demanding the resignation of Shokin, who was a "corrupt prosecutor," she said.