WASHINGTON (Reuters) - Henry Paulson, a Republican who was U.S. Treasury secretary during the 2008 financial meltdown, on Friday called a Donald Trump presidency "unthinkable" and said he will vote for Democrat Hillary Clinton.

Paulson joins a growing list of establishment Republicans who say they will not cast a ballot in the Nov. 8 election for Trump, the party's presumptive nominee and a political neophyte whose populist rhetoric runs counter to many long-held Republican principles.

"When it comes to the presidency, I will not vote for Donald Trump," Paulson, who was chief executive of Goldman Sachs before becoming Treasury chief under Republican President George W. Bush, wrote in an opinion piece in the Washington Post.

"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.

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.

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

"Simply put, a Trump presidency is unthinkable," Paulson said.

The Trump campaign did not immediately respond to a request for comment.

Brent Scowcroft, a national security adviser to two Republican presidents, endorsed Clinton on Wednesday, and Richard Armitage, a deputy secretary of state under George W. Bush, said last week he would support her.

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.

(Reporting by Eric Beech)