WASHINGTON – President Donald Trump and aides plan to tell House Speaker Nancy Pelosi that they will not cooperate with the ongoing impeachment inquiry until the full House votes to authorize it, officials said Friday.

"We'll be issuing a letter," Trump told reporters. "As everybody knows, we've been treated very unfairly, very different from anybody else."

Democrats have said that Trump's refusal to answer subpoenas and provide documents could constitute an impeachable offense in and of itself as they investigate the president's dealings with Ukraine.

Even so, the White House is preparing a letter of objection to Pelosi, said two officials who spoke on condition of anonymity because it has not yet been released. They said they did not know if the letter would be sent on Friday or next week.

White House officials said they want a full House vote on authorizing the impeachment inquiry that Pelosi herself announced last week.

In confirming the letter while speaking to reporters on the South Lawn, Trump demurred when asked if he would ever comply with subpoenas.

“That’s up to the lawyers,” he said, adding that his lawyers have never seen something “so unfair.”

Trump, meanwhile, again defended himself against claims that he pressured Ukraine to investigate Democratic political rival Joe Biden, using the prospect of U.S. aid as leverage.

"I'm only interested in corruption," Trump said. "I don’t care about Biden...I do care about corruption."

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Also under fire for asking China to investigate Biden, Trump tweeted Friday morning that "as President I have an obligation to end CORRUPTION, even if that means requesting the help of a foreign country or countries. It is done all the time. This has NOTHING to do with politics or a political campaign against the Bidens. This does have to do with their corruption!"

Biden has said Trump is the corrupt one, and is asking foreign countries to dig up dirt because he fears the former vice president will defeat him in 2020.

As for being "done all the time," historians and foreign policy analysts said no previous president has asked foreign countries to get involved in American politics the way Trump has.

The administration's letter to Pelosi seeking new rules on the impeachment inquiry is expected to echo a request sent earlier this week by House Republican leader Kevin McCarthy, R-Calif.

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Criticizing "the swiftness and recklessness" of the impeachment drive, McCarthy asked Pelosi if Republicans would be let into the process and if administration lawyers could cross-examine witnesses.

"Unfortunately, you have given no clear indication as to how your impeachment inquiry will proceed – including whether key historical precedents or basic standards of due process will be observed,” McCarthy said in his letter.

In replying to McCarthy, Pelosi said the majority is proceeding under authority it already has.

“There is no requirement under the Constitution, under House Rules, or House precedent that the whole House vote before proceeding with an impeachment inquiry," Pelosi said in a letter to the House Republican leader.

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The back-and-forth reflects the political pressures surrounding impeachment.

White House and Republican officials have said they want to get all House members on the record about whether they favor an impeachment investigation, particularly Democrats who represent districts carried by Trump in 2016.

Republicans also question whether Pelosi and the Democratic leadership could get their impeachment inquiry authorized by the full House.

“The problem is you don't have the votes,” Rep. Doug Collins, R-Ga., told Judiciary Chairman Jerry Nadler, D-N.Y., during a Sept. 17 hearing.

Pelosi has repeatedly said that committees are free to conduct their own impeachment investigation, and "there's no requirement that there be a floor vote." Democrats have also said they suspect the White House will try to stonewall their probe.

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The impeachment investigation centers on Trump's request to Ukrainian President Volodymyr Zelensky to have his government investigate U.S. political rival Joe Biden, a Democratic presidential candidate. Investigators are also looking at whether Trump held up aid to Ukraine until it agreed to investigate Biden and his son, who had business interests in the country.

On Thursday night, a group of House committee chairmen released text messages from diplomats describing an effort to lobby Zelensky into a public pledge to investigate Biden in exchange for the nearly $400 million in aid as well as a private meeting with Trump.

Outside the White House on Friday, Trump focused on a line in the text messages in which the U.S. ambassador to the European Union said there was “no quid pro quo” in the U.S. offers.

“There is no (quid) pro quo,” Trump said, adding, “That was in the text message that I saw and it nullified everything.”

Trump was referring to a message from EU Ambassador Gordon Sondland, who pushed back on a message from Bill Taylor, the top U.S. diplomat in Ukraine. Taylor, in an exchange about withholding assistance to Ukraine, said "it's crazy to withhold security assistance for help with a political campaign."

Sondland responded, saying, "Bill, I believe you are incorrect about President Trump's intentions. The President has been crystal clear no quid pro quo's of any kind."

In a letter to colleagues, the House chairmen also referenced Trump's call on Thursday to have China investigate Biden over his son's business activities in that country.

“This is not normal or acceptable," the letter said. "It is unethical, unpatriotic, and wrong. American Presidents should never press foreign powers to target their domestic political rivals."

Trump has denied wrongdoing, and described the impeachment drive as a "coup" designed to drive him from office.

Contributing: Bart Jansen