President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE asserted in an early morning tweet on Wednesday that none of the Democratic presidential contenders could defeat him in the 2020 general election and that House Democrats are trying to “illegally” impeach him as a result.

“You would think there is NO WAY that any of the Democrat Candidates that we witnessed last night could possibly become President of the United States. Now you see why they have no choice but to push a totally illegal & absurd Impeachment of one of the most successful Presidents!” Trump tweeted.

You would think there is NO WAY that any of the Democrat Candidates that we witnessed last night could possibly become President of the United States. Now you see why they have no choice but to push a totally illegal & absurd Impeachment of one of the most successful Presidents! — Donald J. Trump (@realDonaldTrump) October 16, 2019

Trump called the Democratic candidates "clowns" and claimed the economy would crash if any of them were elected next year.

Our record Economy would CRASH, just like in 1929, if any of those clowns became President! — Donald J. Trump (@realDonaldTrump) October 16, 2019

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Trump’s messages marked his first reaction to the fourth Democratic presidential debate Tuesday night, during which many of the candidates trained their attacks on Sen. Elizabeth Warren Elizabeth WarrenOvernight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds On The Money: Half of states deplete funds for Trump's 0 unemployment expansion | EU appealing ruling in Apple tax case | House Democrats include more aid for airlines in coronavirus package Warren, Khanna request IG investigation into Pentagon's use of coronavirus funds MORE (D-Mass.), who has been rising in the polls.

Impeachment was the first topic discussed at the outset of the debate, with several candidates outlining their support for impeaching Trump.

Trump has lashed out at House Democrats’ impeachment inquiry, claiming his political opponents are seeking to overturn the results of the 2016 presidential election. Trump has taken aim Speaker Nancy Pelosi Nancy PelosiDemocratic senator to party: 'A little message discipline wouldn't kill us' Overnight Health Care: New wave of COVID-19 cases builds in US | Florida to lift all coronavirus restrictions on restaurants, bars | Trump stirs questions with 0 drug coupon plan Overnight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds MORE (D-Calif.) and other top congressional Democrats in addition to his potential 2020 opponents during official White House events, campaign rallies and on Twitter.

Meanwhile, some recent polls have shown Warren or former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE defeating Trump in hypothetical general election match-ups.

House Democrats last month launched an impeachment inquiry focused on Trump’s contacts with Ukraine after it was revealed that he asked the nation's leader to investigation Biden and his son. An anonymous intelligence community whistleblower filed a complaint alleging Trump was using his official office to solicit foreign interference in the 2020 election.

Trump has maintained he did nothing wrong on the call, saying it was focused on fighting "corruption" and not politics. No evidence of criminal wrongdoing related to the Bidens has emerged.

The president has also accused Democrats of a “witch hunt” — using a moniker he often reserved to describe former special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s Russia investigation.

The White House has refused to cooperate with the impeachment inquiry, accusing Democrats of neglecting due process and fairness by refusing to hold a formal vote to begin impeachment proceedings, among other things.

Vice President Pence, the Pentagon and Trump's personal attorney Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE — a key person of interest in the impeachment inquiry — all refused to comply with subpoenas for documents in connection with the impeachment inquiry on Tuesday.

Updated at 9:15 a.m.