Donald Trump's "false bravado" about the 2020 election has been "totally exposed" by the whistleblower scandal, a leading US commentator says.

American political columnist Mark Shields savaged the US President, saying the impeachment process was the correct path after allegations Mr Trump tried to influence the election.

The President is accused of asking his Ukrainian counterpart to investigate former Vice President Joe Biden and the Democratic candidate's son during a phone call with Ukrainian President Volodymyr Zelenskiy.

Democrats say the president violated his oath of office and the reports sparked an impeachment investigation in the House.

During a weekly segment on PBS Newshour with New York Times columnist David Brooks — who argued impeachment was the wrong course to take — Mr Shields said the scandal showed Mr Trump was terrified of losing the election to Mr Biden.

"I mean, what it puts to rest is the lie about the confidence of the Trump campaign: 'We're leading in all polls. We're ahead'," he said.

"He was so terrified, so intimidated, the president of the United States got on the phone with the leader of Ukraine to get dirt on the one Democrat who in every major poll was beating him and that candidate's son.

"I mean, this shows the terror, the intimidation. And the false bravado is just totally exposed."

Mr Brooks argued an impeachment process would overshadow the Democratic primary and give Trump a victory with a widely-expected not guilty verdict in the Republican-dominated Senate.

The growing scandal and impeachment probe has sparked mixed reactions in the US and dominated headlines around the world since it was first reported by the Wall Street Journal on September 21.

Mr Trump has struggled in the polls since his upset election in 2016, never receiving more than a 50 per cent approval rating in aggregated polls.

The latest poll shows 53.2 per cent of Americans disapprove of his presidency, and 42.8 per cent approve.

Polling shows Mr Biden, the leading Democratic candidate, would beat Mr Trump if an election was held today.

The US President went on the offensive on Saturday, ripping into Democratic politicians, labelling them as "do nothings" and "savages."

In Mr Trump's firing line were Democrats Jerry Nadler, Adam Schiff as well as Alexandria Ocasio-Cortez and her "squad".

"Can you imagine if these Do Nothing Democrat Savages, people like Nadler, Schiff, AOC Plus 3, and many more, had a Republican Party who would have done to Obama what the Do Nothings are doing to me," he said. "Oh well, maybe next time!"

"PRESIDENTIAL HARASSMENT!" he added in a follow up tweet, that comes days after Democrat leaders announced an impeachment inquiry.

His critics struck back, calling his tweets anti-Semitic.

The New York Times reported that the whistleblower who brought Mr Trump's alleged actions to light is a CIA officer and an expert on Ukraine.

It was also revealed that the White House tried to "lock down" a transcript of the call with Ukrainian President Volodymyr Zelensky, knowing that him asking for a "favour" would be problematic.

White House is also of severely restricted distribution of memos detailing Mr Trump's calls with foreign leaders, including Russia's Vladimir Putin and Saudi Arabia's Mohammed bin Salman, after embarrassing leaks of his conversations early in his tenure.

Mr Trump's representative for Ukraine, Kurt Volker, also abruptly quit this week.

Ukrainian politicians expressed alarm after Mr Volker resigned.

Ukrainian ex-president Petro Poroshenko called news of Mr Volker's decision "disturbing" and praised his contribution to "strengthening of our strategic partnership with the United States." "It would be hard to overstate his firmness and strategic vision countering the Russian aggression," Mr Poroshenko wrote on Facebook.

"With Kurt, we all were feeling more confident in Ukraine."

Former foreign minister Pavlo Klimkin said the resignation was "not a political one, but a real loss" for Ukraine.

"We were not indifferent to him — this is a truly rare case in politics ... It's all so sad," Mr Klimkin said on Facebook.

US House Speaker Nancy Pelosi, who had been in the fence about pursuing impeachment, said Democrats now had no choice.

"The actions taken to date by the president have seriously violated the Constitution," she said.

The president has insisted that he has done nothing wrong, and son Eric Trump announced that nearly $15 million had been raised from supporters over the issue.