Described only as an intelligence official who once worked at the White House, the whistleblower was the first to raise concerns about Trump's attempt to pressure Ukraine to investigate political rival Joe Biden.
"The Whistleblower got it sooo wrong that HE must come forward," Trump said.
"The Fake News Media knows who he is but, being an arm of the Democrat Party, don't want to reveal him because there would be hell to pay. Reveal the Whistleblower and end the Impeachment Hoax!"
A parade of current and former diplomats and national security officials, however, have since corroborated the essence of the complaint - an apparent attempt to pressure Ukraine to investigate the president's Democratic opponents. At issue is whether Trump ordered the withholding of nearly $400 million in crucial military aid to Ukraine to get its new president to investigate Biden and his son Hunter, as well as a debunked conspiracy theory involving the Democrats in 2016.
The whistleblower's lawyer, Mark Zaid, said on Twitter Sunday that his client had made a direct offer to Republican lawmakers to answer their questions in writing.
The offer, made to Devin Nunes, the ranking Republican on the House Intelligence Committee, underscored the whistleblower's desire that the complaint be handled in a nonpartisan way, Zaid told CBS News. "Recent GOP messaging, led by President Trump (incl this morning), has been to highlight original #WBer & demand disclosure of identity," Zaid said on Twitter.