By Henry M. Paulson Jr.

Republicans stand at a crossroads. With Donald Trump as the presumptive presidential nominee, we are witnessing a populist hijacking of one of the United States' great political parties. The GOP, in putting Trump at the top of the ticket, is endorsing a brand of populism rooted in ignorance, prejudice, fear and isolationism. This troubles me deeply as a Republican, 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.

I'm not the first Republican to say Trump is a phony and should not be president, and I expect there will be many more to come. But as a former chief executive and treasury secretary, I hope to bring an additional perspective to the discussion.

Let's start by talking about his business acumen. When Trump assures us he'll do for the United States what he's done for his businesses, that's not a promise -- it's a threat. The tactics he has used in running his business wouldn't work in running a truly successful company, let alone the most powerful nation on Earth.

Every good businessman or -woman carefully analyzes all the available facts before making a decision. Trump repeatedly, blatantly and knowingly makes up or gravely distorts facts to support his positions or create populist divisions.

He excels at scorched-earth tactics in negotiations during bankruptcy proceedings. Here, the "Art of the Deal" businessman is a master at advantaging himself over his fellow stakeholders and partners. In essence, he takes imprudent risks and, when his businesses fail, disavows his debts. He has branded himself as a business genius by flaunting and exaggerating his wealth. He is adept at leveraging his brand through licensing agreements that enable him to slap his name on anything he can. But while marketing and self-promotion may translate on the campaign trail, they have little relevance in running our country. And although his business dealings have allowed him to increase his inherited wealth, none of us knows by how much -- we have only his word for it.

Now let's talk about Trump the prospective president. Are we to believe that Trump, with his intensely divisive rhetoric and behavior, could bridge our country's partisan divide? The American people are disgusted with business as usual in Washington, and it's not hard to understand why. They feel as though they are being left behind or are afraid that they will be. They aren't getting honest answers, and they believe that the most important problems are not being solved. This is not the fault of one political party; it's the fault of too many partisans and ideologues on both sides who are unwilling or unable to work together.

I can't help but wonder 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. The only reason we avoided another Great Depression was because Republicans and Democrats joined together to vote for the Troubled Asset Relief Program -- a vote that they knew would be politically unpopular but in the best interest of our country. Critical to that effort was the leadership of President George W. Bush. As I led Treasury's efforts to fashion a difficult, imperfect, controversial but essential solution with bipartisan support, I was - and still am - grateful to have had President Bush at the helm.

Today's challenges include economic stagnation and disruptions in the labor markets -- driven to a large extent by technological advances moving at warp speed -- that are widening income disparity, destroying jobs and hollowing out the middle class. And populists on each side are playing to fears and frustrations, pointing fingers at scapegoats and creating boogeymen: blaming the banks, greedy companies or foreigners for our problems. But the politics of grievance is not the answer.

Now is the time for a bipartisan approach to policy solutions that address our most difficult domestic problems. This requires a president who exhibits an ability to compromise -- and basic civility -- neither of which Trump displays.

There are two key principles that the next president must address to maintain our economic competitiveness and security. Populists in both parties are demagoguing these principles, with Trump leading the way.

First, we need to maintain the United States' fiscal strength by reforming entitlements. There's no example of a nation continuing as a great power if its fiscal strength is lost. Anyone, whether Republican or Democrat, who has studied our entitlement programs and can do basic math knows they are unsustainable in their present form. If not fixed soon, they threaten our nation with a debt burden that would undermine the retirement security of young Americans and future generations. It doesn't surprise me when a socialist such as Bernie Sanders sees no need to fix our entitlement programs. But I find it particularly appalling that Trump, a businessman, tells us he won't touch Social Security, Medicare and Medicaid.

Second, we need to welcome rather than shrink from trade and economic competition. Trump calls our current trade deals "disgusting, the absolute worst ever negotiated by any country in the world." This is simply false. According to the Peterson Institute for International Economics, the average American household income is roughly $10,000 higher because of the postwar expansion of trade. Because of trade, we add jobs and foster innovation and competitiveness. That doesn't mean that people aren't losing jobs and suffering in certain industries. However, it is wrong to tell the American people that we can turn back the clock and win, with merely 4 percent of the world's population, by walling ourselves off from the remaining 7 billion people and the markets they represent. Instead, we need to fix the programs that help U.S. industries and workers transition to new and better jobs. We need better training, new education programs and a more robust safety net. The policies Trump endorses would destroy, not save, U.S. jobs.

Simply put, a Trump presidency is unthinkable.

As a Republican looking ahead to November, there are many strong conservative leaders in statehouses across the United States and in Congress, whose candidacies I am actively supporting. They have a big job to do to reinvent and revitalize the Republican Party. They can do so by responding to the fears and frustrations of the American people and uniting them behind some common aspirations, while staying constant to the principles that have made our country great.

When it comes to the presidency, I will not vote for Donald Trump. I will not cast a write-in vote. 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. To my Republican friends: I know I'm not alone.

Henry M. Paulson Jr. is chairman of the Paulson Institute and a former U.S. treasury secretary and chief executive of Goldman Sachs.

