Just minutes after Mitch McConnell swatted away questions about the astonishing feud between Senator Bob Corker and President Donald Trump by saying that “everybody gets to express themselves” in the US, the Republican Senate majority leader witnessed another peer launch a blistering attack on the president.“There are times when we must risk our careers in favour of our principles. Now is such a time,” Jeff Flake, an

Arizona Republican, said in a remarkable retirement speech in which he castigated Mr Trump for undermining US values and for showing “flagrant disregard for truth or decency”.The public enmity between Mr Trump and a handful of congressional Republicans underscores the high stakes for both sides as they both try to push through comprehensive tax reform. “There have often been spats, but the rhetoric in modern times has never been this coarse, personal or sophomoric,” said Tom Daschle, former Democratic Senate majority leader.While Mr Trump is venting ire at Republican leaders and party members he blames for the failure to pass any major legislation, his attacks could come at a heavy price given the Republicans have only a two-vote majority in the Senate to pass tax cuts. Even as Mr McConnell tries to maintain a veneer of unity between his caucus and Mr Trump, Senate Republicans are facing a threat from outside government in Steve Bannon. The former White House strategist is enlisting a slate of candidates to run primary challenges against every Republican senator up for re-election in 2018 except Ted Cruz in Texas.“Establishment Republicans are in full collapse. They’re not even fighting back. They’re out of ideas, guts and out of money,” Mr Bannon told the Financial Times. “Flake was polling like crazy and the numbers were coming back terrible. Flake shows you one important thing. The money is getting turned off. He went down without a fight.”The departures of Mr Corker and Mr Flake represent wins for Mr Bannon who was already revelling in the victory of Roy Moore, a firebrand evangelical, over the establishment candidate in a recent Republican senate primary in Alabama. He had been lining up an opponent for Mr Corker and was championing Kelli Ward, a doctor who unsuccessfully challenged Mr McCain in 2016, to take on Mr Flake next year.Mr Bannon has also been encouraging Erik Prince, founder of Blackwater, the Iraq war-era private security company, to challenge John Barrasso, a moderate Republican senator from Wyoming, in the 2018 midterm elections.Larry Sabato, a University of Virginia politics expert, said Mr Flake was “smart” to retire. “He had a miserable year in front of him — tough primary he might have lost, potentially a difficult general election, too. At best his chances to hold his seat were 50-50,” he said, adding that the Democrats had a “good chance” of taking the seat if Ms Ward became the Republican candidate.That is a concern for Mr McConnell as the Republicans have only a 52-48 Senate majority. Republican leaders worry that failure to pass tax reform will anger their base and make it easier for Mr Bannon’s candidates to oust mainstream Republicans in the 2018 primaries.Doug Heye, former spokesman for the Republican National Committee, said the danger in trying to elect what anti-establishment Republicans call “true conservatives” was that it would help Democrats running for re-election in 2018, such as Claire McCaskill in Missouri and Joe Donnelly in Indiana.“Some have said they would rather see 20 ‘true conservatives’ — however you define that — than a Republican majority,” said Mr Heye. “But how will guaranteeing Claire McCaskill or Joe Donnelly stay in the Senate help make America great again?”Mr Bannon counters that the establishment is misreading voters in the same way it underestimated Mr Trump’s chances of winning the White House. He said the decision by lawmakers such as Mr Flake and Mr Corker, who had little trouble raising cash, underscored how the mood had turned against mainstream lawmakers. “They will start to fall like dominoes,” he said. Mr McConnell also faces uncertainty over whether Mr Trump will back the establishment politicians next year or vent his frustrations with the Republican leadership by campaigning for anti-establishment candidates. At a recent press conference with the Kentucky senator, Mr Trump said he has a “great relationship” with Mr Bannon but was “going to see if we can talk him out of that”. But Mr McConnell has learnt that the president can be quick to abandon his party. Mr Trump was angry after the Alabama primary when he himself had supported Luther Strange, the mainstream candidate, over Mr Moore who was backed by Mr Bannon.