The US public now backs an impeachment inquiry against Donald Trump over allegations he pressured the Ukrainian president to investigate the son of former vice president Joe Biden, a senior Democrat has claimed.

The “tide has completely changed” following the release of a whistleblower complaint about Mr Trump’s conversations with Volodymyr Zelenskiy, said Nancy Pelosi, the Democratic leader of the House of Representatives.

Having spent months resisting calls to press ahead with an investigation, Ms Pelosi decided last week that the US president had crossed a line with his controversial phone call.

Several opinion polls have revealed a slim majority of the public is now in favour of impeachment, with support for the measure jumping between seven and 12 per cent in recent months.

“In the public, the tide has completely changed,” the House Speaker said at an event hosted by the Texas Tribune news website. “It could change now – who knows – but right now after seeing the complaint and the cavalier attitude the administration had towards it, the American people are coming to a different decision.”

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Ms Pelosi said it “doesn’t matter” how the inquiry affected the Democrats’ chances in the 2020 election, adding: “Our first responsibility is to protect and defend the Constitution of the United States.”

She said: “We cannot have a president of the United States undermining his oath of office, his loyalty to his oath of office, undermining our national security, and undermining the integrity of our elections.”

​Democratic presidential candidate Julian Castro also told the event that he believed the public will increasingly back the inquiry.

“After more evidence gets out there... you’ll see more people of different political stripes start to support it," he said.

Mr Trump is accused of withholding military assistance – paid for by taxpayers – to pressure Mr Zelenskiy to launch an investigation of Democratic presidential hopeful Joe Biden and his son, Hunter, who had served on the board of a Ukrainian energy company.

A whistleblower, thought to be a CIA agent who had been assigned to the White House, also made a formal complaint that aides had tried to hide evidence of the 25 July call by moving the record of it to a server reserved for information of “an especially sensitive nature”.

Mr Trump has repeatedly denied any wrongdoing, describing the inquiry as a “witch hunt” and an “impeachment scam” and seeking to switch the focus on to the Bidens.

Ms Pelosi, who has previously claimed attorney general William Barr has “gone rogue” to protect Mr Trump’s presidency, said there was now “a cover-up of a cover-up” in the White House.

“If this activity, this pattern of behaviour were to prevail... then it’s over for the republic,” she said. “We will have the equivalent of a monarchy.”