US President Donald Trump has lashed out at Democrats moves towards his impeachment in a series of angry tweets claiming he has “done more than any president”.

“You don’t impeach presidents for doing a good (great!) job,” he insisted, before accusing his rivals of “treasonous crimes”.

His attack came as Democrats stepped up their impeachment investigation of Trump even as the party’s leaders remained dubious about the political value of the move 14 months before the next presidential election.

The House Judiciary Committee approved new procedures allowing members to demand more documents and testimony from the White House, declaring for the first time that its probe of the president is focused on “impeachment.”

Committee Chairman Jerry Nadler said the measures will serve to enhance an “aggressive series of hearings” aimed at determining whether to recommend articles of impeachment against the president.

“The resolution before us represents the necessary next step in our investigation of corruption, obstruction and abuse of power,” Nadler said before it was approved by the Democrat-controlled committee.

The probe is focused along four lines: allegations that Trump illegally interfered with the Russia election meddling investigation, that he took part in hush payments to alleged former mistresses, that he has used his office to enrich himself and that he offered pardons to government and campaign officials to protect him.

While a significant step closer, there are still doubts about whether Democrats will eventually vote to impeach the president, the political equivalent of formally indicting him for a crime.

Democratic Party leaders, especially House Speaker Nancy Pelosi, have opposed the move as politically risky, as opinion polls show the public generally is against going through the drama of charging the president.

It is virtually certain that the Republican-controlled Senate, which would effectively try the president on the allegations, would acquit, absent new evidence.

Nadler’s committee is planning to interview Trump’s former campaign manager Corey Lewandowski next week, and is seeking to question former White House Counsel Don McGahn.

The White House maintains that Trump’s executive privilege gives him the power to prevent McGahn from testifying.