Democrats stepped up their impeachment investigation of President Donald Trump Thursday 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.

"The conduct under investigation poses a threat to our democracy. We have an obligation to respond to this threat," Nadler said.

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.

(File photo: AP)

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

More than half of the 235 Democrats in the House have endorsed an impeachment investigation, yet the party has been accused of being too cautious and avoiding use of the word "impeachment" itself.

Pelosi, who has indicated she thinks the party should focus on next year's presidential and congressional elections, refrained Thursday from giving impeachment a full-throated endorsement.

Americans "understand that impeachment is a very divisive measure," she said, adding nevertheless that Nadler's investigation would be an important step in bringing to light the evidence against the president.

"I support what is happening in the Judiciary Committee," she said. "Legislate, investigate, litigate -- that's the path that we have been on."

Nadler defended the investigations of Trump being conducted by three separate House committees.

"Some call this process an impeachment inquiry.  Some call it an impeachment investigation. There is no legal difference between these terms, and I no longer care to argue about the nomenclature," he said.

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.

McGahn was a key source for the Mueller investigation into Russia meddling, which mapped out at least 10 instances in which Trump appeared to obstruct or attempt to obstruct justice.