WASHINGTON — Democrats on Tuesday sharply criticized a plan suggested by Sen. Pat Toomey (R., Pa.) that would effectively involve a witness trade in the impeachment trial of President Donald Trump, giving each party the chance to call one witness.

Any such deal would likely lead to testimony from both John Bolton, the president’s former national security adviser, whom Democrats want to hear from, and Hunter Biden, son of former Vice President Joe Biden, whom Republicans hope to question. Democrats quickly rejected the suggestion, though majority Republicans could push it forward on their own.

“What can Hunter Biden tell us about the president’s conduct with Ukraine? What can Hunter Biden tell us about the president’s obstruction of Congress? Nothing," Senate Minority Leader Chuck Schumer (D., N.Y.) said at a news conference Tuesday morning. "Obviously, the bottom line is very simple. They are always looking for a shiny object to divert attention from the facts.”

Schumer later added, “We’re not bargaining with them. We want four witnesses and four sets of documents, then the truth will come out.”

Appearing Tuesday morning on MSNBC and echoing a fellow Democratic senator who made the same argument, Sen. Chris Coons (D., Del.) said: “Why on earth would we engage in that kind of a crazy quid-pro-quo where we trade a relevant witness for an irrelevant witness in the middle of a quid-pro-quo trial?”

The private maneuvering by Toomey and Democratic response comes as the Senate nears formal votes on witnesses, one of the only major remaining uncertainties in a trial that has largely played out as expected. After the White House concluded its opening arguments Tuesday, the Senate could vote on witnesses by the end of the week, deciding whether Bolton or others ever speak under oath about what they know.

The pressure on the GOP has ratcheted up since the New York Times reported Sunday that Bolton, for a forthcoming book, wrote that Trump personally told him he was withholding $391 million in military aid to Ukraine unless the country began an investigation into the Bidens, confirming the central accusation in the impeachment charges.

Bolton, who has said he would testify if subpoenaed, would be the first witness with firsthand information to directly link Trump to the plot. The report about his book has rocked the trial after weeks of Republican resistance to calling witnesses.

“We want the truth,” Schumer said Tuesday, arguing that testimony from Bolton and other top White House officials is critical to understanding Trump’s actions.

Toomey offered fellow Republicans one way out of the jam Monday, privately floating a plan, reported by the Washington Post, to allow one witness for each party. The idea would give wavering Republicans an opening to call Bolton while also putting a spotlight on Hunter Biden — aiding the White House defense.

Even if Democrats reject the plan, the GOP majority could have enough votes to subpoena them both, and many Republicans will face pressure from conservatives to put a Biden under questioning.

Toomey’s office has not commented on the trade proposal.

Democrats cannot call Bolton without support from at least four Republican senators. But it’s not clear how much support Toomey’s idea has within the GOP. Some senators on Tuesday were looking at other options — such as making Bolton’s book available to senators in a classified setting — and others suggested that they still opposed the idea of having anyone testify.

Democrats argue that Hunter Biden is not relevant to the central facts of the case: that Trump used his office to pressure a foreign country to harm Joe Biden’s prospects in the 2020 election, and then blocked Congress’ investigation of the matter.

The Trump team argues the president withheld aid because he was concerned about corruption in Ukraine, particularly Hunter Biden’s high-paid role with an energy company, Burisma, that had previously come under investigation.

Joe Biden, as vice president, led a push to oust a Ukrainian prosecutor while his son sat on the company’s board. Biden was carrying out the official policy of the United States and its European allies in trying to oust a prosecutor noted for being soft on corruption. Hunter Biden was not accused of any wrongdoing, and the investigation into Burisma was dormant at the time.

Toomey has been noncommittal when it comes to whether he will support calling witnesses. He hasn’t closed the door, but has also raised concerns that issuing a subpoena could lead to a legal fight that drags out the trial. And he’s said he sees a “high bar” for taking that step, arguing that any witness would have to have information so compelling it might change votes on the verdict .

“The criteria is not ‘Does the witness have something interesting to say?’" Toomey said Friday in an interview with The Inquirer and the Allentown Morning Call. "The criteria is, ‘Are they likely to be able to shed definitive light on a disputed issue that is so central to this case that the resolution of it could change my final conclusion?’ So, that’s a high bar.”

He also argued that Hunter Biden is a relevant witness.

“The Democrats’ argument ... largely rests on this notion that the only possible explanation for the president’s wishing to have an investigation is the president’s personal corruption, that there is no other possible explanation,” Toomey said.

On Monday, hours after the Bolton news broke, a spokesman said that “Sen. Toomey’s position remains unchanged” and that he would decide about calling witnesses after Trump’s attorneys conclude and senators ask their questions. Toomey did not answer reporters’ questions Tuesday.