Developments in the Robert Mueller probe – which hit a milestone Wednesday with the sentencing of former Trump fixer Michael Cohen – have congressional Democrats openly revisiting the possibility of impeachment or even future prosecution of the president.

On the former, top Democrats in recent days have gone so far as to say the campaign finance violations Cohen claims President Trump directed amount to an “impeachable offense.” Some lawmakers swiftly raised the issue of the president’s culpability after Cohen was sentenced to three years in prison.

Yet key Democrats also have sought to make the distinction that it’s not clear whether the alleged offenses are so serious yet as to justify impeachment.

“I think what these indictments and filings show is that the president was at the center of … several massive frauds against the American people,” incoming House Judiciary Committee Chairman Jerry Nadler, D-N.Y., told CNN’s “State of the Union” on Sunday. But he clarified, “You don't necessarily launch an impeachment against the president because he committed an impeachable offense.”

Figures like Nadler are sure to face rising pressure from the liberal base, and influential activists like Tom Steyer, to pursue impeachment proceedings in the new Congress.

The president fired a warning shot in an interview with Reuters, maintaining he’s done nothing wrong.

“I’m not concerned [about impeachment], no. I think that the people would revolt if that happened,” he told Reuters.

The impeachment debate has focused on an evolving set of alleged or suspected offenses since the appointment of Special Counsel Mueller. First, there was the suspicion of collusion between Russia and Trump campaign associates. Then, Democrats monitored the investigation’s turn to look at possible obstruction of justice. Most recently, they’ve seized on allegations made by Cohen – and echoed by federal prosecutors – that Trump ordered Cohen to make hush-money payments to two women going into the 2016 presidential campaign.

Cohen pleaded guilty to campaign finance violations in connection with those payments, among numerous counts for which he was sentenced Wednesday.

But Trump has adamantly denied legal culpability in those transactions, tweeting that they did not amount to illicit campaign contributions – and even if they did, it would amount to a civil case.

“Lawyer’s liability if he made a mistake, not me … Cohen just trying to get his sentence reduced,” Trump tweeted.

“I don’t think they have a violation of the law,” Trump lawyer Rudy Giuliani also said.

But top Democrats are now citing all three of these areas – suspicion of Russia collusion, obstruction of justice and campaign finance violations – as subjects to explore in the new year.

The political and logistical challenge for Democrats considering impeachment will remain even after the party takes control of the House in January: Republicans control the Senate, and it takes a two-thirds majority in that chamber to convict an impeached president.

Nadler, speaking on MSNBC last week, cited that hurdle in arguing that the House would need to have convincing evidence of serious offenses to proceed with impeachment.

At the same time, Democrats are looking at easing the path to potentially prosecute Trump in the future.

Nadler acknowledged that, while he disagrees with the finding, the Justice Department is bound by an opinion that a sitting president can’t be indicted for crimes – so he is considering introducing legislation that would effectively extend the statute of limitations so a sitting president could be prosecuted for potential offenses after leaving office.

The legislation specifically would put the statute of limitations on hold while a president is in office.

“You should not have a system where a president, anybody, is above the law,” he said.

Giuliani told Politico that such a move would “violate the spirit if not the letter of the constitutional protection against ex post facto legislation” – or punitive legislation that applies retroactively.

Sen. Chris Van Hollen, D-Md., speaking Wednesday on Fox News’ “America’s Newsroom,” echoed Nadler in saying grounds for impeachment could exist.

“Whether that’s grounds to do it and whether you actually pursue impeachment is a whole [other] question,” he added.

Van Hollen said until the full facts emerge, “people are not itching to get into an impeachment of President Trump.”

Fox News’ John Roberts contributed to this report.