Mark Zaid, a lawyer for the whistleblower, wrote on Twitter that the legal team was working with both parties in Congress and "we understand all agree protecting whistleblower's identity is paramount".
Republicans have generally stood behind Trump, but in an administration beset by near-constant turnover and turmoil, some former White House officials began to venture criticism.
"The objective of this letter is to formally notify you of serious concerns we have regarding our client's personal safety", the letter says, adding that recent comments by Trump are reason for "heightened" concern. "If he continues to focus on that white whale", Bossert added, "it's going to bring him down".
Nixon resigned before impeachment proceedings were launched.
A rough transcript released by the White House shows Trump repeatedly pushed Ukrainian President Volodymyr Zelensky to investigate former Vice President Joe Biden, his potential 2020 political rival, and his son, Hunter Biden.
After the disclosures about Trump's calls to the Australian and Mexican leaders, a White House adviser raised the possibility of lie detector tests for the small number of people in the West Wing and elsewhere with access to transcripts of Trump's phone calls.
She pointed out that in the U.S., 'our elections are the fundamental point of our democracy'.
Specifically, he wanted Ukraine to probe Joe Biden's role in the dismissal of the country's prosecutor-general, Viktor Shokin, who was axed in 2016.
No evidence of wrongdoing has surfaced regarding either of the Bidens.
He derided Schiff, who has served almost two decades in the House, as an "illegitimate chairman" who "wants to hang the president". Democrats are planning a rapid start to their push for impeachment, with hearings and depositions starting this week.
Other topics in the cartoons this week include the United Nations climate conference, where Swedish teenager Greta Thunberg berated world leaders for not keeping their promises on reducing greenhouse gases; vaping bans; and New York City Mayor Bill de Blasio's decision to drop out of the Democratic presidential primaries.
Schiff has said the whistleblower has agreed to testify, but the logistics involving security had yet to be set. They said they expect the situation to become even more risky for their client and any other whistleblowers, as Congress seeks to investigate this matter.
In a day of bitter partisan skirmishing, President Trump's allies and his Democratic antagonists took to the nation's talk shows Sunday to present stridently opposing views of the impeachment inquiry that hit Washington last week like a tornado, sweeping almost all other priorities aside.
There remains a stark partisan divide on the issue, with 88% of Democrats approving and 93% of Republicans disapproving of the inquiry.
The California Democrat later said it was a "parody" of the call. At the same time, Democrats' campaign arm was mobilizing to support the candidates, according to a person on the call who spoke on condition of anonymity to discuss the details.
But Trump's staunchest congressional supporters - including Sen.
"I know the difference between a whistleblower and a "deep state" operative".
Democrats have said articles of impeachment - formal charges - against Mr Trump could be completed in as little as a month, and swiftly voted on in the House, where the party has a majority.