Newly empowered House Democrats kicked off a barrage of investigations into President Donald Trump's taxes, real estate business and Russia ties Thursday, raising the pressure on the White House after two years of insulation by a Republican majority.

Trump lashed out about "presidential harassment" as one House committee opened a hearing into forcing the release of his long-hidden tax returns, and another began questioning his controversial policy to separate immigrant children from their parents at the US-Mexico border.

"The Dems and their committees are going 'nuts,'" Trump tweeted.

"The Republicans never did this to President Obama, there would be no time left to run government," he said.

Both sides were girding for tough legal clashes pitting congressional powers against those of the presidency.

House Judiciary Chairman Jerrold Nadler threatened Trump's acting attorney general, Matthew Whitaker, with a subpoena if he did not turn over records of his communications with the White House in reference to Special Counsel Robert Mueller's ongoing investigation into Russian election meddling.

And Adam Schiff, the chairman of the House Intelligence Committee, laid out a plan to begin probing Trump's business ties to Russia, citing fears of money laundering and of "foreign financial or other leverage over President Trump."

Schiff's committee is planning to hear testimony from the president's former personal lawyer, Michael Cohen, who has already divulged to investigators inside information about the operations of the president's real estate business, the Trump Organization and Trump's 2016 election campaign.

House Democrats have a long list of Trump-focused issues to probe after being stifled for two years by majority Republicans.

Winning control of the House in November allows them to open an investigative assault on the White House in the same way that -- contrary to Trump's claim -- Republicans laid siege to former president Barack Obama.

A shortlist of topics include conflicts of interest and mismanagement by cabinet members; Trump's benefits from family business while in the White House; and security lapses by Trump and his White House staff.

"We will not surrender our constitutional responsibility of oversight," House Speaker Nancy Pelosi said Thursday.

Altogether, the probes elevate the chance that Trump could face impeachment if serious wrongdoing is found.

Thursday's House Ways and Means hearing into Trump's taxes would break what he declared in 2017 as a red line -- that investigations should not touch his business and finances.

Unlike previous presidents, Trump has refused to release his tax records. His lawyers claim they have been under review by the Internal Revenue Service since 2002. A 2016 New York Times analysis based on limited information concluded that Trump may have paid minimal or even no taxes each year since 1995.

Schiff's Intelligence Committee is planning a deep-dive into Trump's financial and business ties to Russia, suggesting that they could involve money laundering.