A growing realization that the unpredictable New York real estate developer is in a position to win the Republican nomination and then battle Hillary Clinton for the White House in November's election has caused some investors to sell U.S. stocks. They fear having such a wild-card president could trigger trade wars, hurt the economy and add a lot of market volatility.

"As the market rarely feasts on lack of predictability - Trump represents a nightmare for investors this year," said hedge fund manager Douglas Kass of Seabreeze Partners Management Inc, who said last week that he was adding to his existing short bet on the U.S. stock market in part because of Trump's increasingly strong position in the race.

Trump's statements on business and Wall Street don't neatly fit into one ideological worldview, but if anything, they are seen as isolationist in a globally connected world. He can also suddenly pick on businesses over various issues, such as his call for a boycott of Apple Inc's (AAPL.O) products after the tech giant refused to help the FBI unlock the iPhone used by one of the San Bernardino shooters.