McFarland told the Senate Foreign Relations Committee in written comments that she "was not aware" of any communications between Flynn and the Russian ambassador — an assertion that appears to be contradicted by court documents unsealed Friday
after Flynn pleaded guilty to lying to the FBI
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.
Maryland Sen. Ben Cardin, the top Democrat on the Senate Foreign Relations Committee, told CNN he thinks McFarland's nomination is imperiled.
"I think there should be no action on this nomination until we get the information," Cardin said.
Foreign Relations Chairman Bob Corker of Tennessee said that he didn't anticipate McFarland would have to return to the committee, but he acknowledged there now was a "major pause" on her nomination.
"There's no question that this is going to create a major pause in her nomination. I mean, that's at a minimum," Corker said. "I doubt we'll bring her back to the committee but I know that, you know, people will have questions about what happened, but I don't think we will do a committee hearing on it."
He added later, "If she did testify inappropriately, that's a big, big problem. I just don't know ... but look: it's a problem. And her nomination is frozen for a while until that gets out."
CNN has reached out to a McFarland representative for comment on the hold and has not yet received a response.
McFarland's confirmation is in doubt over her role during the Trump transition and as the deputy to Flynn at the National Security Council. She left the National Security Council to take the ambassador post in Singapore not long after Flynn resigned as national security adviser.
Court documents laying out the charge against Flynn state that he spoke to a senior transition team official about what to discuss with Kislyak after the Obama administration issued new sanctions against Russia. McFarland was not mentioned in the court filings, but CNN has reported, citing sources familiar, that she was the transition official who spoke to Flynn.
Her written questions to the Senate Foreign Relations Committee did not acknowledge that conversation.
"Did you ever discuss any of General Flynn's contacts with Russian Ambassador Sergey Kislyak directly with General Flynn?" Sen. Cory Booker asked McFarland in July in written correspondence with the committee, which is overseeing her nomination.
"I am not aware of any of the issues or events as described above," McFarland wrote.
The foreign relations panel approved McFarland's nomination in September, but it has not moved for a floor vote.
Her nomination is the longest pending State Department nomination for the administration -- 173 days and counting, tied with a nominee for an undersecretary position selected on the same day as McFarland.
CORRECTION: This story has been updated to correctly identify Sen. Bob Corker.