Democrats say they don’t have the answer to those questions yet. That’s why they were before the D.C. Circuit of the U.S. Court of Appeals on Monday, insisting a lower court ruling in their favor should be allowed to kick in so they can see Mueller’s most sensitive grand jury materials.

“This is something that’s unbelievably serious and it’s happening right now, very fast,” Letter said. He cited a heavily redacted portion of the Mueller report that suggests inconsistencies between what the president told Mueller and what former Trump 2016 campaign chairman Paul Manafort told investigators.

“The Manafort situation shows so clearly, very sadly, that the president might have provided untruthful answers,” Letter said.

At least publicly, Democrats have largely trained their impeachment focus on Trump’s attempts to pressure Ukraine into investigating the president’s political opponents. That’s what prompted last month’s historic floor vote setting the ground rules for a series of public hearings.

Appearing on CNN soon after the arguments concluded, Rep. Jamie Raskin (D-Md.), a key member of the Judiciary Committee, said Trump’s honesty could end up being included among the impeachment articles against him.

"Of course, it’s a crime to lie to federal prosecutors in the course of a federal proceeding,” said Rep. Jamie Raskin (D-Md.). “That's perjury. It was also the basis for the GOP-controlled House's impeachment of Bill Clinton for lying under oath, for committing perjury. So it's a very serious offense, and it's obviously something that we take seriously.”

Rep. Jamie Raskin (D-Md.) said President Donald Trump’s honesty could end up being included among the impeachment articles against him. | Jacquelyn Martin/AP Photo

House attorneys have had Mueller on their mind for months as lawmakers proceed with their impeachment proceedings.

In a late September court filing, House lawyers claimed that they needed to see the grand jury materials to flesh out whether Trump lied about his knowledge of contacts with WikiLeaks as it dumped embarrassing Democratic emails during the 2016 campaign.

“Not only could those materials demonstrate the president’s motives for obstructing the special counsel’s investigation, they also could reveal that Trump was aware of his campaign’s contacts with WikiLeaks,” the House lawyers wrote.

A few weeks later, during oral arguments before U.S. District Court Judge Beryl Howell, Letter again made the point.

"I can't emphasize enough — it’s not just Ukraine," he said, adding that the Judiciary Committee "could easily" adopt articles of impeachment against Trump that deal with the Mueller-related themes of obstruction of justice and election interference.

Democrats got more fodder about what’s behind the Mueller report redactions during Roger Stone’s trial in Washington, D.C., which culminated last Friday in a guilty verdict against the longtime Trump adviser for lying to Congress and tampering with one of its witnesses during its Russia probe.

During the Stone trial, jurors heard direct testimony and saw dozens of texts and email messages showing Trump’s campaign closely tracking Julian Assange’s plans to upend Hillary Clinton’s White House aspirations by dumping damaging documents on his site, WikiLeaks. That included at least three conversations between Stone and Trump on the topic that would appear to contradict the president’s written answer to Mueller.

“I do not recall discussing WikiLeaks with [Stone], nor do I recall being aware of Mr. Stone having discussed WikiLeaks with individuals associated with my campaign,” Trump wrote.

Trump attorney Jay Sekulow shrugged off the latest threat to the president’s tenure. “Read the answers to questions. They speak for themselves,” he said in a text message.