With a Senate impeachment trial likely just days away, President Trump and Democrats are locked in a cycle of mutual trolling over whether the trial should include testimony from witnesses.

Democrats are pressing Republicans to accept witnesses. They've made public a list of four, led by former national security adviser John Bolton. Some Republicans are countering that the Senate should summon Joe Biden's son, Hunter Biden, whose suspicious paychecks from a corrupt Ukrainian company were part of the beginning of the Trump-Ukraine affair.

On Sunday morning, after Pelosi appeared on ABC to announce that "this president is impeached for life" — no trolling there, none at all — the president responded that she, too, should have to testify before the trial.

Beyond the substantial question of whether witnesses would add much to the public's knowledge of the matter, Democrats see the issue as a tool to embarrass Republican lawmakers and perhaps damage their chances of reelection in November. Minority Leader Chuck Schumer has pledged to force GOP senators to vote on each witness.

Many Republicans are "very, very worried about going home and saying they're not for witnesses and documents," Schumer said last week. "We are telling our Republican colleagues: You can run, but you can't hide."

That led some in the GOP to suggest Schumer dial it back. "I'm a bit baffled why Democrats are so enthusiastic about forcing witness votes to embarrass GOP senators," former top McConnell aide Josh Holmes tweeted Monday. "That seems like a terrific way to end up with Hunter Biden in the chair."

Indeed, a case can be made that Biden's testimony would be valuable. "In a conventional trial, Biden would be a relevant defense witness," law professor Jonathan Turley wrote recently. The president's motive is part of the Ukraine matter. If Biden's story, and the public knows almost nothing about it, raises reasonable concerns about corruption in Ukraine, then the president's defenders could argue that Trump's view had a real basis.

It is not at all clear that 51 Republican senators, much less any Democrats, would agree to call Biden, but there is no doubt he will be part of the argument.

One key figure in the Ukraine affair, however, appears to have faded from view, even as impeachment reaches a climax. The whistleblower who started the whole investigation remains anonymous. The details of the whistleblower's actions by which the whistleblower brought the Ukraine matter to the attention of the intelligence community to House Democrats, and ultimately to the world, remain unclear.

Democrats, who once hoped to feature public testimony by the whistleblower, argue that the account is no longer needed given the other evidence which they say proves Trump pressured Ukrainian President Volodymyr Zelensky to investigate Hunter Biden and the 2016 election. The whistleblower would have nothing to add, Democrats say, and could suffer recriminations and possibly physical danger if the whistleblower's identity were known.

On the other hand, is the Senate really going to try the president and take a vote on his removal on the basis of an investigation with such murky origins?

It's not an academic question. The whistleblower, in the Aug. 12 complaint, created a template that Democrats used throughout the House Ukraine investigation. "I have received information from multiple U.S. Government officials that the President of the United States is using the power of his office to solicit interference from a foreign country in the 2020 U.S. election," the whistleblower wrote. "Namely, he sought to pressure the Ukrainian leader to take actions to help the President's 2020 reelection bid."

Repeat that a million times, and you have the Democratic impeachment investigation. Has any single person been more influential in crafting the case against the president? Are senators expected to pass judgment on that case without knowing how it began?

And what about the whistleblower's motives? Last August, the Intelligence Community inspector general, Michael Atkinson, said the whistleblower showed "some indicia of an arguable political bias ... in favor of a rival political candidate." In October, Atkinson told the House Intelligence Committee that the whistleblower had a "professional relationship" with one of the 2020 Democratic presidential candidates, according to three knowledgeable sources.

Are senators supposed to vote without knowing anything about that?

Yes, say Democrats. During the House investigation, Intelligence Committee Chairman Adam Schiff strictly forbade any talk that might suggest the whistleblower's identity. When National Security Council official Alexander Vindman testified, Democrats jumped in to bar questions on who Vindman talked to about the Trump-Zelensky call. They were worried it might reveal the whistleblower.

In the past, some influential Republicans rejected that view altogether. "I consider any impeachment in the House that doesn't allow us to know who the whistleblower is to be invalid," Senate Judiciary Committee Chairman Lindsey Graham said in October. "It's impossible to bring this case forward in my view without us knowing who the whistleblower is and having a chance to cross-examine them about any biases they may have. So, if they don't call the whistleblower in the House, this thing is dead on arrival in the Senate."

They didn't call the whistleblower in the House. And look for Democrats to do everything they can to block calling the whistleblower to testify in the Senate.

Is that only because Democrats are concerned about the whistleblower's well-being? Or is there some other reason Democrats do not want the world to know about the origins of the case they propose to use to remove the president from office?

Senators, and the rest of the country, have a right to know the answer.

