United States President Donald Trump blasted Democrats on Thursday as they prepared to launch a barrage of investigations into his Russia ties, real estate business and long-hidden taxes.

Empowered by their election takeover of the House of Representatives, Democrats were to hold two hearings on Thursday focused on Trump's tax returns and his controversial policy to separate immigrant children from their parents at the US-Mexico border.

Trump's acting attorney general Matthew Whitaker is scheduled to appear before the House Judiciary Committee on Friday.

The committee is threatening a subpoena if he refuses to testify and hand over communications with the president related to Special Counsel Robert Mueller's investigation into Russian election meddling. House Intelligence Committee Chairman Adam Schiff is staffing up to expand their probe into Trump's political and business ties to Russia, with Trump's former personal lawyer Michael Cohen scheduled to testify in late February.

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

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

"PRESIDENTIAL HARASSMENT! It should never be allowed to happen again!"

Probes heighten impeachment threat

After being stifled for two years by majority Republicans, the Democrats won control of the House in November 2018, allowing them to open an investigative assault on the White House in the same way that — contrary to Trump's claim on Thursday — Republicans laid siege to president Barack Obama.

Those probes would add to the legal and political burden facing the president from Mueller's probe into possible collusion between the 2016 Trump campaign and Russia, and a Justice Department investigation in New York into various activities of the campaign and Trump Organisation, his umbrella company.

Together, 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.

The Chairman of the committee, Representative Richard Neal, has the power to review privately anyone's tax returns, and possibly make them public. But demanding Trump's tax records from the Treasury could lead to a pitched legal battle over presidential prerogatives.

Business ties to Russia

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

"The president's actions and posture towards Russia during the campaign, transition, and administration have only heightened fears of foreign financial or other leverage over President Trump," Schiff said in a statement on Wednesday.

Cohen, who was a senior executive in the Trump Organisation, was to be the committee's first witness on Friday in a closed session, before he begins a three-year prison sentence for tax evasion and other crimes he said were undertaken at Trump's bidding.

But on Wednesday Schiff announced cryptically that Cohen's testimony would be delayed until February 28 "in the interests of the investigation".

The committee has decided to send transcripts of its closed-door interviews last year with Trump campaign associates to the Mueller investigation, amid suspicions that some may have lied about their Russia ties.

Trump bashed Schiff in a tweet on Thursday.

“So now Congressman Adam Schiff announces, after having found zero Russian Collusion, that he is going to be looking at every aspect of my life, both financial and personal, even though there is no reason to be doing so. Never happened before!” he said.