Former National Security Council official Fiona Hill, on the fifth day of House Intelligence Committee impeachment hearings, appealed to Republican lawmakers to "not promote politically driven falsehoods that so clear advance Russia's interests."
The unfounded theory that Ukraine - not Russia - meddled in the 2016 elections lies at the heart of President Donal Trump's alleged bid to improperly pressure Kiev to open investigations into his Democratic opponents.
Mr Trump asked Ukraine President Volodymyr Zelensky in a July 25 phone call to open investigations into former Democratic vice president Joe Biden - Mr Trump's possible opponent in the 2020 presidential election - and into the 2016 meddling story.
Democrats pursuing Mr Trump's impeachment allege he froze aid to Ukraine and held up a requested high-profile meeting with Mr Zelensky to extract an announcement from Kiev that it would pursue the investigations.
'Fictional narrative'
But Ms Hill suggested that Mr Trump had bought into the Moscow-concocted Ukraine conspiracy theory that was also being pushed by his personal lawyer, Rudy Giuliani.
"This is a fictional narrative that has been perpetrated and propagated by the Russian security services themselves," she told the hearing.
"These fictions are harmful even if they are deployed for purely domestic political purposes," she said.
Ms Hill said she and other US foreign policy officials had pushed hard to disabuse President Trump of the notion that Ukraine and not Russia had meddled in the elections three years ago.
The false Ukraine story included the unfounded charge that Democrats hid a secret computer server controlled by the cybersecurity firm Crowdstrike in Ukraine that would prove the Ukraine meddling theory.
Impeachment witnesses have described Mr Trump as obsessed by the theory.
In the July 25 phone call, Mr Trump asked Mr Zelensky for a "favour," saying: "I would like you to find out what happened with this whole situation with Ukraine," he said, mentioning Crowdstrike by name.
"The server, they say Ukraine has it."
Republicans say the story justified Mr Trump's demand for an investigation by Kiev.
"President Trump had good reason to be wary of Ukrainian election meddling against his campaign and of widespread corruption in that country," Devin Nunes, the senior Republican on the Intelligence committee, said in Thursday's hearing.
Ms Hill said, however, that there were no grounds to the theory, but that Mr Trump was listening to Mr Giuliani and not the government's Russia and Ukraine experts.
She recounted that John Bolton, the White House national security advisor at the time, said Mr Giuliani "was a hand grenade who going to blow everyone up."
By that, Mr Bolton meant Mr Giuliani "was pushing issues and ideas that would come back to haunt us. And that's where we are today," she told the panel.
Damaging testimony
Ms Hill's comments came on the heels of explosive testimony from a top US diplomat Wednesday that the White House had indeed set a precondition of opening the investigations for a Zelensky summit with Mr Trump.
Gordon Sondland, the ambassador to the European Union, told the panel that Mr Trump ordered him and other diplomats to follow Mr Giuliani's lead on Ukraine policy.
He then said that Mr Giuliani's requests to Ukraine to open the investigations targeting Democrats "were a quid pro quo for arranging a White House visit for President Zelensky."
He also said he interpreted the July 18 freeze of $391 million in security aid to Ukraine as equally designed to pressure Kiev for the investigations.
Ms Hill was appearing on the last day of scheduled impeachment hearings at the Intelligence Committee.
While a committee official declined to comment on whether they would hold call more witnesses or not, they are expected to soon wrap up this investigation phase of the process.
They will then send their evidence to the House Judiciary Committee to produce formal charges - articles of impeachment - against Trump.
The Judiciary Committee could itself hold more hearings, including seeking testimony from Trump or his representatives.
