UNITED States President Donald Trump has said the White House is preparing a letter to House of Representatives speaker Nancy Pelosi formally objecting to the Democrats’ impeachment inquiry without an official vote.

It is expected to say the administration will not co-operate with the probe without that vote – but the president also said he believes it will pass.

Trump acknowledged that Democrats in the House “have the votes” to begin a formal inquiry but he claimed the move will backfire politically. “I really believe that they’re going to pay a tremendous price at the polls,” he said.

Pelosi last week announced that the House was beginning the inquiry but did not seek the consent of the full chamber, as was done for impeachment investigations into former presidents Richard Nixon and Bill Clinton.

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Trump's allies have suggested for days that without a formal vote, the House is merely conducting standard oversight, entitling legislators to a lesser level of disclosure from the administration. The Justice Department raised similar arguments last month, though that was before Pelosi announced the impeachment investigation.

Trump also denied there was a quid pro quo as he pressed the Ukrainian government to investigate Democratic rival Joe Biden, and said he did not do it for political purposes.

The president asked Ukraine and China to launch probes into the former vice-president and 2020 Democratic hopeful, alleging, without evidence, that there was misconduct by Biden and his son Hunter.

Trump claimed his call was not political, because “I never thought Biden was going to win” the primary. He added of rooting out corruption: “I actually feel I have an obligation to do that.”

In a letter to House minority leader Kevin McCarthy, Pelosi said there is “no requirement” for the whole House to vote before an impeachment inquiry.

Allan Lichtman, a history professor at American University, said there is no clear-cut procedure in the constitution for such a move, leaving many questions about possible presidential obstruction untested in court. “There’s no specification in the constitution in what does and does not constitute a more formal impeachment inquiry or investigation,” he said.

Rudy Giuliani, the president’s personal lawyer, dismissed the premise of the inquiry, which is centred on Trump asking Ukraine to investigate Biden.

“The president was not tasking Ukraine to investigate a political opponent,” He said. “He wanted an investigation into a seriously conflicted former vice president of the United States who damaged the reputation of the United States in Ukraine.”