Democrats launched a sweeping new probe of US President Donald Trump on Monday, an aggressive investigation that threatens to shadow the president through the 2020 election season with potentially damaging inquiries into his White House, campaign and family businesses.

House Judiciary Committee Chair Jerrold Nadler said his panel was beginning the probe into possible obstruction of justice, corruption and abuse of power and is sending document requests to 81 people linked to the president and his associates.

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The broad investigation could be setting the stage for an impeachment effort, although Democratic leaders have pledged to investigate all avenues and review special counsel Robert Mueller's upcoming report before trying any drastic action.

Nadler said the document requests, with responses to most due by March 18, are a way to "begin building the public record".

"Over the last several years, President Trump has evaded accountability for his near-daily attacks on our basic legal, ethical, and constitutional rules and norms," said Nadler (a Democrat from New York).

"Investigating these threats to the rule of law is an obligation of Congress and a core function of the House Judiciary Committee."

'Ridiculous'

Trump dismissed the Nadler probe and others as futile efforts "in search of a crime".

"Ridiculous!" he exclaimed.

White House Press Secretary Sarah Sanders called the House probe "a disgraceful and abusive investigation into tired, false allegations".

In a statement on Monday night, Sanders said: "Chairman Nadler and his fellow Democrats have embarked on this fishing expedition because they are terrified that their two-year false narrative of 'Russia collusion' is crumbling. Their intimidation and abuse of American citizens is shameful."

Separate congressional probes are already swirling around the president, including an effort announced on Monday by three other House Democratic chairs to obtain information about private conversations between him and Russian President Vladimir Putin.

In a letter to the White House and State Department, the House intelligence, Foreign Affairs and Oversight and Reform panels sent broad requests for details about Trump and Putin's private meetings by phone and in person.

In addition to document requests, the committees are asking to interview interpreters who sat in on meetings, including a one-on-one session in Helsinki last year.

The State Department pledged to "work cooperatively with the committees".

The new probes signal that now that Democrats hold a majority in the House, Trump's legal and political peril is nowhere near over, even as the special counsel's Russia investigation winds down.

They are also an indication of the Democrats' current strategy - to flood the administration with oversight requests, keeping Trump and his associates on trial publicly while also playing a long game when it comes to possible impeachment.

While some more liberal members of the Democratic caucus would like to see Trump impeached now, Democratic leaders have been more cautious.