George W. Bush's former treasury secretary has publicly endorsed Hillary Clinton, saying that a Donald Trump presidency would be 'unthinkable'.

In a double blow to Trump, Henry Paulson's comments came as George Will, a prominent conservative columnist, announced he was quitting the Republican party in protest.

Mr Paulson, who worked in the Bush administration during the 2008 financial meltdown, has joined a growling list of establishment Republicans who will not cast a ballot for their party's presumptive nominee.

Paulson called Trump a 'populist' who was 'hijacking one of the United States' great political parties with a campaign rooted in 'ignorance, prejudice, fear and isolationism'.

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Henry Paulson, George W. Bush's former treasury secretary, has publicly endorsed Hillary Clinton, arguing that a Donald Trump presidency would be 'unthinkable'

Paulson, a Republican, called Trump a 'populist' who was 'hijacking one of the United States' great political parties with a campaign rooted in 'ignorance, prejudice, fear and isolationism'

'This troubles me deeply as a Republican', he wrote in an op-ed for the Washington Post. 'But it troubles me even more as an American.'

'Enough is enough. It's time to put country before party and say it together: Never Trump.'

Paulson accused Trump, who has touted his business acumen as a real estate developer during his campaign, of taking 'imprudent risk' and then disavowing his debts when ventures fail.

'Every good businessman - or woman - carefully analyzes all the available facts before making a decision,' he wrote in the piece, published Friday.

'Trump repeatedly, blatantly and knowingly makes up or gravely distorts facts to support his positions or create populist divisions.'

Paulson also took aim at Trump's opposition to trade agreements, which Paulson said have created US jobs and fostered innovation and competitiveness.

'We need to welcome rather than shrink from trade and economic competition,' he wrote.

'Trump calls our current trade deals "disgusting, the absolute worst ever negotiated by any country in the world." This is simply false.'

Paulson cited statistics from the Peterson Institute for International Economics which show that the average American household income is now $10,000 higher because of 'postwar expansion of trade'.

Paulson, who worked with Bush (pictured together) during the 2008 financial meltdown, has joined a growling list of establishment Republicans who will not cast a ballot for the party's presumptive nominee

Paulson said he will be voting for Clinton in the hope that she can 'bring Americans together'

'Because of trade, we add jobs and foster innovation and competitiveness,' he writes.

Paulson, who helped steer a $700 billion bailout of the financial system through Congress during the financial meltdown, said Trump is a 'phony' who is unfit to be president.

'I can't help but think what would have happened if a divisive character such as Trump were president during the 2008 financial crisis, at a time when leadership, compromise and careful analysis were critical,' he said.

Instead, Paulson writes, the next president needs to be able to take a 'bipartisan approach' to the nation's most difficult domestic problems.

'This requires a president who exhibits an ability to compromise and basic civility,' he continues. 'Neither of which Trump displays.'

Conservative commentator and columnist George Will revealed on Saturday he is quitting the Republican party because of Trump's rise

But Paulson does believe presumptive Democratic nominee Hillary Clinton does display those characteristics, and revealed that is the reason why he will be voting for her come November.

'I'll be voting for Hillary Clinton, with the hope that she can bring Americans together to do the things necessary to strengthen our economy, our environment and our place in the world,' he said.

'To my Republican friends: I know I’m not alone.'

Paulson's op-ed comes just two days after Brent Scowcroft, a national security adviser to two Republican presidents, publicly endorsed Clinton.

And last week Richard Armitage, a deputy secretary of state under George W Bush, threw his support behind the Democratic candidate as well.

Trump's rise has caused conservative commentator and columnist Mr Will to leave the Republican Party entirely.

Will said he is now an 'unaffiliated voter' in the state of Maryland but would not reveal if he planned to vote for Clinton or a third-party candidate when he made his announcement on Friday.

But Will said a Trump presidency backed by a Republican-led Congress would be worse than a Clinton presidency with a Republican-led Congress, he told PJ Media.

Will said his message for conservatives was to make sure Trump is defeated in November.