Washington (CNN) The legal team for the whistleblower whose account kicked off the impeachment inquiry is preparing for the possibility that lawmakers will call their client to testify in the Senate, two people familiar told CNN.

The White House counsel's office remains in discussion with Republican senators about the length and scope of President Donald Trump's likely Senate impeachment trial . But the whistleblower's lawyers are preparing for all possibilities, including receiving a subpoena for testimony as allies of the President continue their demands to hear from the anonymous person who ignited the Ukraine controversy.

A subpoena would put the whistleblower in uncharted territory, and raise the risk that his or her identity could come out in the course of the trial in the Senate -- where, unlike in the Democrat-led House, some Republicans have expressed a desire to hear from the whistleblower.

Trump, too, has repeatedly demanded he or she come forward during the impeachment proceedings.

House Democrats have charged Trump with two articles of impeachment: abuse of power and obstruction of Congress, to set in motion the third impeachment of a US president in history. Democrats say Trump abused his office by pressuring Ukraine to investigate his political rival Joe Biden and his son ahead of the 2020 election while withholding a White House meeting and $400 million in US security aid from Kiev. And they say that Trump then obstructed the investigation into his misconduct with a blanket blockade of subpoenas and refusing to allow key senior officials to testify before Congress.

Depending on how senators structure their likely trial, a subpoena could compel the whistleblower to cooperate with anything from written questions to a closed-door interview in a secure location to a public hearing that would expose the whistleblower's identity.

The whistleblower's legal team is looking at historical precedent and conducting research to see what kind of case it could mount to block the whistleblower from testifying, if it came to that. One source said the legal team is closely monitoring lawmakers' public statements about their client and is taking note of recent reporting that some Republicans have pushed back on White House demands for testimony.

The offer for the whistleblower to answer written questions from senators still stands for the upcoming trial, the people familiar said.

A senior GOP aide said this is a possibility if 51 senators supported it, but like all other ideas for conducting the trial, it remains under discussion among lawmakers and White House officials.

Senate Majority Leader Mitch McConnell and Minority Leader Chuck Schumer are expected to meet in the days ahead to negotiate a the scope of the trial, including its length and whether witnesses will testify.

White House officials have been engaging Senate Republicans to feel out how they might influence the process and keep it from getting dragged out, even as Trump himself itches for a theatrical fight to clear his name.

Still, Senate lawmakers have already begun investigating some issues that relate to the impeachment process. The Senate Intelligence Committee has quietly interviewed witnesses involved in the handling of the whistleblower complaint, which detailed Trump's conduct on a July 25 phone call with Ukrainian President Volodymyr Zelensky, CNN has learned.

Senate lawmakers are examining how key events in the months before intelligence watchdog Michael Atkinson's transmitted the complaint to Congress -- including the departure of former intelligence director Dan Coats in August, failed nomination of his potential replacement, Rep. John Ratcliffe, and naming of acting Director of National Intelligence Joseph Maguire -- may have affected the disclosure process, a committee source said.

The committee has interviewed witnesses about that issue and indicated it may want to hear more from officials who played a part of the disclosure process, including Atkinson, Maguire and officials at both Office of the Director of National Intelligence and CIA, several sources told CNN. The panel's chairman, Republican Sen. Richard Burr, has publicly stated that the committee's focus is on "process" and not the content of the complaint itself, and sources have reaffirmed that those questions remain "at the core" of their inquiry.

Top Republican senators, meanwhile, are signaling they may call no witnesses at all when the trial begins next month. GOP Sen. Lindsey Graham said Sunday he did not want to "turn the Senate into a circus" by calling Democratic Rep. Adam Schiff, as President Trump has demanded, and McConnell said Tuesday he envisions one scenario in which no trial ever occurs.

"The House managers would come over and make their argument. The President's lawyers would then respond. And at that point, the Senate has two choices: It could go down the path of calling witnesses and basically having another trial. Or it could decide -- and again, 51 senators could make that decision -- that they've heard enough and believe they know what would happen and could move on the two articles of impeachment sent over to us from the House," McConnell told reporters before noting that no decision has been made.

Senate Intelligence Committee Chairman Richard Burr has consistently said he wants to hear from the whistleblower for his panel's investigation. The whistleblower's attorneys have ruled out an in-person interview in any context. Trump has also said he wants House Speaker Nancy Pelosi and the Bidens to testify -- a nonstarter with most lawmakers, even Republicans -- but one Democratic source said it was clear from McConnell's Monday press conference that the majority leader "is worried about witnesses."

"He's stuck between Trump and a hard place," the source added.

The whistleblower's legal team started preparing for the possibility of being called in the Senate trial before Thanksgiving in anticipation that the process will likely start in January, one source said.

Democrats are likely to resist thrusting the whistleblower back into the spotlight, however, given that House Democrats plan to present evidence independent of what the whistleblower brought to light.

"These calls to identify the whistleblower and have them be a witness, I find outrageous and irresponsible. There is no earthly reason that the whistleblower needs to participate at this time because all he/she did was report wrongdoing. If there's an investigation as a result of this complaint, the investigation itself reveals whether or not the complaint was valid," Sen. Angus King, an independent who caucuses with Democrats and a member of the Senate Intelligence Committee, told CNN in November.

"If a person's walking by a house and sees flames coming out of the upper story windows and calls the fire department, the fire department determines whether there's a fire and then commences to fight it. It makes a call is irrelevant," he added.