(CNN)Hank Paulson, the Republican Treasury secretary during the 2008 financial crisis, has endorsed Hillary Clinton, adding his name to prominent GOP heavyweights who are backing the presumptive Democratic nominee.
Paulson's support comes amid mounting concerns about Donald Trump's candidacy. Paulson, a controversial figure given President George W. Bush's handling of the downturn, said "a Trump presidency is unthinkable" and predicted that other conservatives would rally behind her.
"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," Paulson wrote in The Washington Post. "To my Republican friends: I know I'm not alone."
Paulson, once the CEO of Goldman Sachs, criticized Trump's business record, divisiveness and brand of populist rhetoric.
Clinton has been criticized for close ties to Wall Street, but not many Republicans of Paulson's stature have formally endorsed her.
"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," Paulson wrote.
Paulson's support comes days after Brent Scowcroft, a top national security adviser to Republican presidents dating back to Gerald Ford, similarly backed Clinton, and Robert Kagan, a prominent Republican neoconservative, is headlining a fundraiser for Clinton, Foreign Policyreported on Thursday. Last week, Richard Armitage, who served in the State and Defense Departments under Bush and President Ronald Reagan, announced he will vote for Clinton.