The White House is preparing to formally object to the House impeachment inquiry and put lawmakers on notice they would receive no cooperation from 1600 Pennsylvania Ave. unless they take a formal vote on proceeding, President Trump said Friday.

Trump told reporters the White House is preparing a letter to House Speaker Nancy Pelosi, who announced the impeachment inquiry last week. She has said the investigation can go forward without a formal vote.

The investigation was triggered by the administration handling of a whistleblower report related to a July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky in which Trump asked Zelensky for a “favor.” Just days earlier, Trump had frozen $400 million in military aid to Ukraine, which has been fighting Russia-backed separatists for five years.

Kenneth F. McCallion, a former federal prosecutor and expert in Russian organized crime, racketeering and counterintelligence operations, said the president can't ust refuse to cooperate.

"Claims of executive privilege cannot be broadly used by the executive branch to stonewall all subpoenas for information and testimony from witnesses employed by the White House or any of the executive departments," McCallion said. "Congress ... has a [constitutional] duty of oversight over the executive branch, which it can only properly exercise through either voluntary cooperation or by subpoena."

McCallion noted the third article of impeachment against Richard Nixon charged him with contempt of Congress for failing to respond to House subpoenas.

Trump predicted the impeachment inquiry would backfire on Democrats.

“I really believe that they’re going to pay a tremendous price at the polls,” Trump said, adding it would be up to White House lawyers when it comes to cooperating with lawmakers.

Meantime, Trump was busily trying to discredit the investigation on Twitter. Trump said the whistleblower had gotten the facts wrong and that House Intelligence Committee Chairman Adam Schiff had failed to disclose the whistleblower had reached out to a committee aide before the complaint was submitted.

He also said his sole goal in seeking an investigation by Ukraine into former U.S. Vice President Joe Biden, a leading 2020 Democratic presidential contender, was to rout corruption, not to gain any political advantage.

“As president, I have an obligation to end corruption, even if that means requesting help of a foreign country or countries,” Trump tweeted.

Schiff fired back, saying Trump is under the impression it’s his right to ask a foreign nation to help him politically – something Federal Election Commissioner Ellen Weintraub has noted repeatedly is illegal.

He added Trump’s request to Zelensky posed a threat to national security and threatened democracy itself.

On Thursday, Trump invited China to open its own inquiry into Biden. The suggestion comes just days before high-level trade talks aimed at resolving a tariffs war are about to resume. He doubled down on that taunt Friday but denied tying a trade deal to any investigation of unsubstantiated allegations Biden’s son, Hunter, persuaded China’s national bank to invest $1.5 billion in an investment firm on whose board he sat. The younger Biden did not become a member of the firm’s board until after his father left office.

Sen. Mitt Romney, R-Utah, the 2012 GOP presidential nominee, tweeted it strains credulity that Trump's requests were anything but political.

"By all appearances, the President’s brazen and unprecedented appeal to China and to Ukraine to investigate Joe Biden is wrong and appalling,"

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House investigators set a Friday deadline for Secretary of State Mike Pompeo to turn over documents related to Ukraine.

Senate Majority Leader Mitch McConnell of Kentucky is tying his re-election campaign to the impeachment inquiry, pledging in Facebook ads he will work to prevent Trump’s removal from office. If Trump is impeached by the House, he then would be tried in the Senate.

The Senate never has voted to remove a president from office. There have been three previous impeachment proceedings: Andrew Johnson, Nixon and Bill Clinton. Nixon resigned before he could be tried.