Mitt Romney, the 2012 Republican presidential candidate, has launched an excoriating attack on President Donald Trump in an opinion piece for the Washington Post.

In a sign of how unhappiness with Trump's performance is increasing among senior Republicans, Romney – still regarded as a potential challenger for the highest US office – says Trump has 'not risen to the mantle of the office'. December marked a 'deep descent', he says, citing the departures of senior staff and the 'abandonment' of US allies.

Romney, who will be sworn in tomorrow as senator for Utah, gives credit to Trump for some of his trade policies, his reform of criminal justice and his appointment of conservative justices.

However, he says a president should 'demonstrate the essential qualities of honesty and integrity, and elevate the national discourse with comity and mutual respect'. Trump's 'shortfall' in this area has been 'glaring', he says.

Romney points to the 'dismay' caused around the world by Trump's words and actions, citing polls showing confidence that he would do the right thing in world affairs had fallen from 84 per cent to only 16 per cent in Germany, Britain, France, Canada and Sweden.

He mourns the lack of American leadership, calling for 'the highest office once again acting to inspire and unite us'.

He said: 'I do not intend to comment on every tweet or fault. But I will speak out against significant statements or actions that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions.'

Romney concluded with an expression of optimism about the future for the US, saying: 'The people of this great land will eschew the politics of anger and fear if they are summoned to the responsibility by leaders in homes, in churches, in schools, in businesses, in government – who raise our sights and respect the dignity of every child of God – the ideal that is the essence of America.'