President Donald Trump has accused Democrats in the US House of Representatives of launching a "fishing expedition in search of a crime".
He was referring to their inquiry into his alleged obstruction of justice and abuse of power.
On Monday, documents were requested from 81 people and groups, including some of the president's closest allies.
They include some of Mr Trump's aides and campaign staff, family members and business associates.
Mr Trump tweeted his response on Tuesday.
The greatest overreach in the history of our Country. The Dems are obstructing justice and will not get anything done. A big, fat, fishing expedition desperately in search of a crime, when in fact the real crime is what the Dems are doing, and have done!— Donald J. Trump (@realDonaldTrump) March 5, 2019
A second tweet published by the president shortly afterwards reads simply: "PRESIDENTIAL HARASSMENT!"
The House of Representatives Judiciary Committee oversees the US justice system, including federal law enforcement agencies.
Its Democratic chairman, Jerry Nadler, said in a statement: "This is a critical time for our nation, and we have a responsibility to investigate these matters and hold hearings for the public to have all the facts.
"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. Investigating these threats to the rule of law is an obligation of Congress."
President Trump has previously dismissed Mr Nadler's probe as a "political hoax".
In a separate development on Tuesday, New York State insurance regulators announced they were issuing subpoenas to the Trump Organization's longtime insurance broker.
The move by the New York Department of Financial Services comes after Mr Trump's ex-personal lawyer Michael Cohen testified to Congress and alleged that multiple tax and finance crimes were directed by Mr Trump.
And Congressman Bill Pascrell, a New Jersey Democrat who sits on the powerful Ways and Means Committee, said his party would formally demand Mr Trump's tax returns this month, report US media.
What is this latest round of investigations about?
Democrats took control of the House of Representatives in January.
More than five House committees are now investigating alleged attempts by Russia to meddle in the 2016 election campaign, the president's tax returns and potential conflicts of interest involving Mr Trump's family.
Those inquiries are in addition to the investigation by special counsel Robert Mueller, who is expected to file his report into Russian election interference and associated matters soon.
Mr Nadler's committee has the power to conduct impeachment hearings, which can ultimately lead to a president's removal.
He said it was "very clear" that the president had obstructed justice and indulged in other abuses of power but added that it was too early to discuss removing the president from office.
Who received requests?
Trump Organization chief financial officer Allen Weisselberg, long-time Trump personal assistant Rhona Graff, ex-White House counsel Don McGahn and current Trump personal lawyer Jay Sekulow are on the list of names released by the House Judiciary Committee.
Donald Trump Jr is also on the list. He has previously been forced by members of Congress to answer questions about a June 2016 meeting at Trump Tower, in which he sat down with a Kremlin-linked lawyer who had offered dirt on Mr Trump's opponent Hillary Clinton.
Others on the list include another of Mr Trump's sons, Eric, his son-in-law Jared Kushner, former aides Steve Bannon and Hope Hicks, and former Attorney General Jeff Sessions.
Wikileaks and its founder Julian Assange have also been sent documents requests by the committee. The organisation, which publishes secret information and news leaks provided by anonymous sources, published emails stolen from the Democratic National Committee and released them during the 2016 campaign, causing political embarrassment to Mrs Clinton and her campaign team.
Roger Stone, Mr Trump's confidant, who is currently under investigation by Special Counsel Mueller for allegedly lying to prosecutors about his communications with Wikileaks during the presidential campaign, has also received a request from the committee.
After Mr Stone's arrest in January, he denied that he had played a role as an intermediary between the campaign and Wikileaks.
The documents would be used by the committee to determine which witnesses to call to testify in the coming months, a counsel for the House Judiciary Committee said.
If they do not comply within the next few weeks, subpoenas ordering them to attend will be issued.
With his announcement, Mr Nadler is taking a scattergun approach to his investigations of the president, the BBC North America reporter Anthony Zurcher says.
He says the press release announcing the investigations doesn't mention Russia or its alleged election meddling by name.
But the key components of the collusion allegations - the 2016 Trump Tower meeting, the WikiLeaks email dumps, changes to Ukraine policy in the Republican Party platform, and the Trump-Russia business dealings - are all over the document requests.