Every Republican in Congress has “beef” with Donald Trump but they are too afraid to challenge him, a political science professor has claimed.

Cal Jillson, a professor at Southern Methodist University, has said that Republicans could turn on the president if they see him as a threat to their own party or election chances.

“For the president's partisans in Congress, it's 'our guy on his worst day is better than your guy on his best day',” Mr Jillson said.

“They stick with him to get the judicial appointments, the tax cuts.”

He added: “Everyone among the Republicans in Congress has a beef with the president but they're afraid of him. If he weakens, that fear will subside.”

However, increased political polarisation in recent years means that most Republicans are expected to stick with Mr Trump “as long as their own status is not in danger”, according to the professor.

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In 1974, three top Republican leaders in Congress famously visited Richard Nixon at the White House to inform him that he faced near-certain impeachment over the Watergate scandal.

Nixon, who had fought the scandal for months, announced his resignation the next day.

More than 40 years later, a highly-polarised Washington is not expected to come to a bipartisan agreement on the issue of impeachment, especially with Mr Trump having strong support within the Republican Party.

“In the past in the US, party members would dissociate themselves from disgraced leaders in order to preserve the party and their own reputations,” Professor Nick Smith, who teaches ethics and philosophy at the University of New Hampshire, said.

“But now President Trump seems to have such a personal hold on the party – more like a cult leader than a US president – that the exits are closed as the party transforms into his image.”

Kathleen Hall Jamieson, director of the University of Pennsylvania’s Annenberg Public Policy Centre, added that with the death of John McCain last year, there is no Republican figure with enough power to challenge Mr Trump.

“Who would go and be credible with Donald Trump, so that he would listen?” she said.

“Mitt Romney? Mitch McConnell? Lindsey Graham? Trump will turn on any of them the minute they say something uncongenial.”

The president attacked Mr Romney, the Republican presidential nominee in 2012, on Saturday for criticising his call for China to investigate his 2020 election rival Joe Biden.

“When the only American citizen President Trump singles out for China’s investigation is his political opponent in the midst of the Democratic nomination process, it strains credulity to suggest that it is anything other than politically motivated,” Mr Romney wrote on Twitter.

In response, Mr Trump referred to the senior Republican as a “pompous ass” who “never knew how to win”.