An email exchange obtained by The New York Times indicates that Ms. McFarland was aware at the time of a crucial December 29 phone call between Mr. Flynn and Mr. Kislyak that was intercepted by American intelligence.
Trump earlier this year nominated McFarland, a former deputy national security adviser, to be the US envoy to Singapore. But the trip comes amid news that his former national security adviser Michael Flynn pleaded guilty to lying to the Federal Bureau of Investigation.
McFarland reportedly wrote in the email that sanctions then-President Obama levied on Russian Federation in response to Moscow's election meddling would make it more hard for Trump to improve relations with Russian Federation, "which has just thrown the United States of America election to him".
The Democratic hold on McFarland's nomination means that Senate Majority Leader Mitch McConnell would have to take procedural steps in order to overcome the hold and confirm her nomination, which would eat up valuable floor time. Prosecutors said he is now cooperating with special counsel Robert Mueller's investigation.
The Times reported that it was unclear whether the transition official was saying she believed that the election had been thrown.
The discrepancy is likely to add to mounting troubles for the White House that stem from Mr. Flynn's interactions with Russian officials.
One of those questions, from New Jersey Democrat Cory Booker, was this: "Did you ever discuss any of Gen. Flynn's contacts with Russian Ambassador Sergey Kislyak directly with Gen. Flynn?"
Court documents released on Friday, along with Flynn's guilty plea, indicate that senior members of Mr Trump's transition team were well aware of his discussions with the Russian ambassador about the Obama administration's sanctions.
U.S. President Donald Trump's nominee for ambassador to Singapore K.T. McFarland testifies at the Senate Foreign Relations Committee hearing on her nomination on Capitol Hill in Washington, U.S. July 20, 2017.
A White House lawyer said on Friday that Ms McFarland did not mean Russian Federation had tipped the election, only that Democrats were portraying it that way. An Associated Press on Friday identified McFarland as a senior transition official mentioned in court documents who discussed with Flynn in 2016 the US sanctions against Russian Federation and potential retaliation from Moscow.
Ms McFarland worked so closely with Flynn on the transition team that her colleagues sometimes referred to her as his "brain".