Republican senators are becoming increasingly unnerved by Donald Trump’s conduct, as national support for impeachment rises.

Democrats have launched a formal impeachment inquiry, over concerns the president pressured Ukraine to investigate Joe Biden and his son.

The ensuing scandal has placed Republicans in a politically precarious situation.

“Everyone is getting a little shaky at this point,” said Brendan Buck, who was previously an aide to Paul Ryan, the former House Speaker.

“Members have gotten out on a limb with this president many times only to have it be cut off by the president.

Everyone Trump has fired or forced out Show all 13 1 /13 Everyone Trump has fired or forced out Everyone Trump has fired or forced out John Bolton Trump claimed to have fired Bolton, his national security adviser, while Bolton claimed he offered to resign. An anonymous White House source that Bolton's departure came as a result of the national security adviser working too independently of the president AFP/Getty Everyone Trump has fired or forced out Anthony Scaramucci Scaramucci lasted only six days in his role as Trump's communications director before being fired by John Kelly, the incoming chief of staff Getty Everyone Trump has fired or forced out Rick Perry Rick Perry announced his resignation just as he became embroiled in the president's impeachment scandal. The White House said Mr Perry was asked by Donald Trump to work with Rudy GIuliani in regards to Ukraine. AP Everyone Trump has fired or forced out Rex Tillerson Tillerson, Trump's first secretary of state, was fired after a series of clashes with the president over policy Getty Everyone Trump has fired or forced out James Mattis Mattis served as secretary of defense from the beginning of Trump's administration until retiring on 1 January 2019. However, the president later claimed that he had "essentially fired" Mattis Getty Everyone Trump has fired or forced out James Comey Comey was fired as director of the FBI early in Trump's presidency after serving in the role for four years prior. His dismissal is widely thought to have been related to the Russia investigation Getty Everyone Trump has fired or forced out Reince Priebus Priebus, Trump's first chief of staff, was forced out after six tumultuous months AFP/Getty Everyone Trump has fired or forced out David Shulkin Veterans affairs secretary Shulkin claims that he was fired, the White House claims that he resigned Getty Everyone Trump has fired or forced out John Kelly Kelly, Trump's second chief of staff, was forced out after 17 months in office. His departure was a confused affair though it is clear that Trump wanted Kelly out AFP/Getty Everyone Trump has fired or forced out Michael Flynn Flynn lasted 24 days as Trump's national security adviser before being fired for lying to the FBI Getty Everyone Trump has fired or forced out Lee Cisna Cisna served as director of citizen and immigration services between October 2017 and June 2019 before being asked to resign amid a major personnel change in the department of homeland security Everyone Trump has fired or forced out Madeline Westerhout Westerhout served as Trump's personal assistant after leaking private information about his family AFP/Getty Everyone Trump has fired or forced out Mira Ricardel Ricardel was forced out of her role as Deputy National Security Advisor after first lady Melania Trump publicly called for her to be fired

“They know he’s erratic, and this is a completely unsteady and developing situation,” he added, according to The Washington Post.

According to the newspaper, few Republicans want to strongly defend the White House publicly, because they doubt the credibility of the president’s claims.

Mr Trump has called on China and Ukraine to investigate Joe Biden, the Democratic 2020 frontrunner, despite the fact that it is illegal in the US to ask for a foreign country to interfere in an election.

A whistleblower complaint about the president’s conduct was met with outrage by Democrats and lies at the heart of the impeachment inquiry.

A second complaint has reportedly been made, intensifying the inquiry.

If the House votes for impeachment, the process then moves to the Senate, currently controlled by a majority of 53 Republicans, to 47 Democrats.

Recent polls show an increase in Americans supporting impeachment, with 48 per cent of those polled backing the measure, and 46 per cent opposing it.

Despite this, just three Republicans from the senate have publicly criticised the president over his behaviour.

Susan Collins, a senator for Maine, condemned the president’s actions as “completely inappropriate”.

“I thought the president made a big mistake by asking China to get involved in investigating a political opponent,” she said.

Ms Collins did not declare support for the president’s impeachment but said she hoped the process “will be done with the seriousness that any impeachment proceeding deserves.”

Mitt Romney and Ben Sasse have also spoken out against Donald Trump.

Mr Romney, the senator for Utah, said the president’s appeal to China and Ukraine was “wrong and appalling” in comments made last week.

In a statement to the Omaha World-Herald newspaper, Ben Sasse, senator for Nebraska, echoed Mr Romney’s condemnation.

“Hold up: Americans don’t look to Chinese commies for the truth,” he wrote.

“If the Biden kid broke laws by selling his name to Beijing, that’s a matter for American courts, not communist tyrants running torture camps.”

On Thursday a voter confronted Joni Enrst, senator for Iowa, for “not standing up” to the president.

“You still stand there silent. And your silence is supporting him in not standing up,” Amy Haskins, an Iowa resident, said.

“Where is the line? When are you guys going to say ‘Enough,’ and stand up and say, ‘You know what? I’m not backing any of this.”

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Ms Ernst replied: “The president is going to say what the president is going to do.”

Some senators, conscious of widespread support for Mr Trump among Republicans, have struck a more cautious note.

Marco Rubio, the senator for Florida, dismissed the president’s China comments as a joke.

“I don’t know if that’s a real request or him just needling the press, knowing that you guys were going to get outraged by it,” he told a group of journalists.

The silence from most Republicans have angered Democrats, who believe the president’s behaviour constitutes a brazen abuse of power.

“My Republican colleagues’ silence seems unsustainable and inexcusable, given the threat to our national security as well as the integrity of our democratic institutions,” said Richard Blumenthal, the senator for Connecticut.

During the Senate stage of an impeachment process, senators conduct a trial presided over by the Supreme Court’s chief justice.

If two-thirds of senators find the president guilty of impeachable offences, he is removed and the vice president replaces him.