Donald Trump might not voluntarily leave the White House if the Democrats fail to win by “big” in 2020, House speaker Nancy Pelosi has suggested.

The US president could contest the result and refuse to give up power if her party’s candidate won by only a narrow margin, she said.

“We have to inoculate against that, we have to be prepared for that,” she told The New York Times, without revealing any legal or political strategy for dealing with the potential scenario.

She referred to similar worries she had in the lead up to the 2018 midterm elections. “If we win by four seats, by a thousand votes each, he’s not going to respect the election,” she said, referring to Mr Trump.

She added: “He would poison the public mind. He would challenge each of the races; he would say you can’t seat these people.”

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Ms Pelosi has been sharing her concerns about the possibility Mr Trump could dispute results of future elections with colleagues in recent weeks, the newspaper reported.

She said she believes the Democrats should “own the centre left, own the mainstream” during the 2020 campaign, as she remains resistant to launching impeachment proceedings against the president over the obstruction of justice case laid out in the Mueller report.

The leading Democrat has been reluctant to even discuss impeachment so far, taking a cautious, step-by-step strategy to her party’s standoff with the White House over obtaining testimony and documents.

Trump rants about Democratic investigations within the Mueller report

Ms Pelosi did note this past week that obstructing congress was one of the articles of impeachment against former president Richard Nixon. “Impeachment is never off the table, but should we start there?” she said on Friday. “I don’t agree with that.”

Democrats are considering other options for dealing with attorney general William Barr, the current focus of their ire, over his refusal to provide an unredacted version of the Mueller report.

Jerrold Nadler, the House Judiciary Committee chairman, has given Mr Barr a Monday deadline to comply with a subpoena demanding the full report, along with its underlying evidence, or face a contempt charge.

Mr Barr could face another subpoena to appear before Mr Nadler’s committee after skipping a hearing last week in a dispute over the rules for questioning him. Mr Nadler has also subpoenaed testimony from former White House counsel Don McGahn.

Democratic presidential hopeful Beto O’Rourke said he believed “there’s enough evidence now for the House of Representatives to move forward with impeachment”.