A Republican billionaire entrepreneur runs for president on a platform of huge tax cuts for the rich and for business and yet, when his candidacy appears ascendant, the markets swoon.

When the prospects brighten for his Democratic challenger — who has not held a private sector job in more than two decades and who is proposing tax increases for the wealthy — stocks climb.

This is yet another paradox of the tumultuous candidacy of Donald J. Trump: He has spent his entire career among business executives, and yet that constituency is voting with hard cash that he should not be president.

I understand that Mr. Trump has pitched his campaign more to white working-class Americans. But no Republican presidential hopeful in memory has been so unpopular in the business community, as one of the prime thermometers of the private sector — the stock market — indicates.