WASHINGTON — Democrats in the House have, so far, been able to keep the Trump whistleblower from testifying and to protect his identity, but when a trial begins in the Senate, where Republicans have the power, that all could change.

House Republicans have formally requested to subpoena — and tried to out the identity of — the individual who filed an anonymous whistleblower complaint alleging President Trump withheld military aid from Ukraine until the country agreed to investigate the Biden family. Those allegations have been supported by private and public testimony from officials, leading Democrats to say the identity of the whistleblower is now irrelevant. House Intelligence Committee Chair Adam Schiff has shut down Republican attempts to out or subpoena them, as some of Trump's allies have advanced conspiracy theories about the whistleblower.

But if the House votes to impeach Trump, the matter moves to the Senate for a trial, and several Republicans say the Senate needs to compel the whistleblower to testify.

“I want to hear from the whistleblower, I think that’s most important,” said North Dakota Sen. Kevin Cramer.

In the event of a trial, Senate Majority Leader Mitch McConnell and Minority Leader Chuck Schumer will meet to try to hash out the rules for the trial, including who gets interviewed and when. A majority of senators is needed for a successful resolution.

Many Republicans will be pushing McConnell to add the whistleblower’s name to the witness list. Sens. Lindsey Graham and Rand Paul have made outing the whistleblower a rallying cry in TV appearances and at Trump rallies. Sen. Thom Tillis said Thursday he would not vote for any resolution that shields the whistleblower from taking part.

“That’s the original accuser, right? What normally happens in any court of law? You hear from the original accuser,” said Tillis.

Federal whistleblowers are granted anonymity under the Whistleblower Protection Act, and lawyers for the Trump whistleblower say outing this individual would deter future whistleblowers from coming forward. They also argue the identity of their client is irrelevant because the contents of the complaint have now been backed up by administration officials.

McConnell and Schumer have not yet begun negotiating impeachment proceedings, and that likely won’t happen until the House inquiry wraps up. Subpoenaing the whistleblower as a public witness is unlikely, because Democrats and even several Republicans support protecting that person’s identity. But some senators have floated another option — letting Trump’s lawyers depose the whistleblower in a closed-door session.

“If the prudent thing to do is keep the person anonymous, I don’t mind that,” said Cramer. “If a deposition by the president’s lawyers is adequate, you could even do it under the authority of the Judiciary Committee or something.”

Republicans who want to hear from the whistleblower have some leverage — McConnell will want to get as many Republicans on board as possible, to avoid the optics of passing a set of rules with more Democratic votes than GOP votes. McConnell himself has not weighed in on whether he thinks the whistleblower should testify.

If a majority of senators cannot agree on a manager’s resolution, the proceedings will be in the hands of Supreme Court Chief Justice John Roberts, who acts as judge of the impeachment trial. Roberts would lay out the schedule, though his decisions could still be overruled by a majority of senators.

House Republicans submitted a witness list that included former vice president Joe Biden and his son, Hunter Biden. They have focused their investigation on conspiracy theories centering around Hunter Biden’s Ukrainian business dealings, and unproven allegations that Ukraine tried to interfere in the 2016 election to aid Hillary Clinton.

If 51 or more Republicans agree, that line of investigation could get taken up on the Senate side, leading to two separate trials playing out — one involving Trump’s alleged quid pro quo with Ukraine, and another involving the Biden family.

Asked what that would look like to the American people, Cramer said, “It’ll look like things are a mess. But I think they’ve drawn that conclusion already.”