Court filings on Friday in cases that stemmed from a federal probe into Russian activities during the 2016 presidential election pointed to potential problem areas for Mr Trump, including whether he instructed six-figure payments to two women during the campaign to keep quiet about affairs.
Federal prosecutors sought prison time for longtime Trump “fixer” Michael Cohen for paying off an adult film star and a former Playboy model at Trump’s behest, evading taxes and lying to Congress about a proposed Trump Organization building in Moscow.
Democratic US Representative Jerrold Nadler has said it would be grounds for impeachment if the payments are proven to be felony campaign finance violations.
“Well, they would be impeachable offenses. Whether they are important enough to justify an impeachment is a different question,” said Mr Nadler, who will lead the Judiciary Committee when Democrats take control of the House of Representatives in January.
Under US law, campaign contributions, defined as things of value given to a campaign to influence an election, must be disclosed.
Such payments are also limited to $2,700 per person.
The White House did not immediately return a request for comment. Press secretary Sarah Sanders said on Friday that Mr Cohen has lied repeatedly and that the filing was insignificant.
Friday’s court filings also revealed new information about contacts between people working for Mr Trump and Russians in the cases of Mr Cohen, Mr Trump’s former longtime personal lawyer, and Paul Manafort, Mr Trump’s short-lived campaign chairman who was convicted in August on tax and bank fraud charges.
Special Counsel Robert Mueller said Mr Manafort lied to investigators about his interactions with a Russian tied to Russian intelligence services.
Mr Mueller’s office said the lying prompted prosecutors last week to retract a plea agreement with Mr Manafort on two separate conspiracy charges.
“I think what these indictments and filings show is that the president was at the centre of a massive fraud – several massive frauds – against the American people,” Mr Nadler said.
Mr Trump said the filings did not prove any collusion with Russia and called for an end to the investigation.
However, the end of the Mueller probe could be the beginning of bigger problems for Mr Trump.
“There’s a very real prospect that on the day Donald Trump leaves office the Justice Department may indict him, that he may be the first president in quite some time to face the real prospect of jail time,” Representative Adam Schiff, the Democrat who will lead the House Intelligence Committee next year, said.
Legal experts are divided over whether a sitting president can be charged with a crime, as well as on whether a violation of campaign finance law would be an impeachable offense.
Republican Senator Rand Paul warned against over-criminalizing campaign finance violations, saying that errors in disclosures should be punished with fines, not jail.
Republican Senator Marco Rubio told CNN he was waiting for the results of the Russia investigation and related federal probes. However, he cautioned that “no one should be above the law.”
Mr Rubio said it would be a “terrible mistake” if Mr Trump pardoned Mr Manafort, saying that “could trigger a debate about whether the pardon powers should be amended.”
Mr Trump has not ruled out a pardon for Mr Manafort, praising him as a good man. In contrast, he has said Mr Cohen, who has cooperated with federal prosecutors, should go to jail.
Mr Schiff said Mr Trump may end up needing to seek a pardon for himself from the next US president.
House Democrats have promised an array of investigations into Mr Trump’s activities.
“What is clear also is that the Republican Congress absolutely tried to shield the president,” Mr Nadler said. “The new Congress will not try to shield the president.”