Add the juggernaut that is Donald J. Trump to the list of what-ifs that is worrying Wall Street.

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 products after the tech giant refused to help the FBI unlock the iPhone used by one of the San Bernardino shooters.

"The election this year is the height of uncertainty," said Phil Orlando, a senior portfolio manager and chief equity strategist at Federated Investors in New York, which manages $351 billion. He said political concerns - personified by Trump's emergence as a frontrunner - are one of the main reasons why he began reducing equity exposure in mid-January.

There are, of course, plenty of other factors having an impact on U.S. financial markets. U.S. stocks rallied on Tuesday after strong U.S. factory and construction data suggested the economy was regaining momentum. That was even as investors contemplated expectations that Trump would do very well in 11 states holding Republican primary or caucus elections on this Super Tuesday.

Little Policy Substance

Trump's rhetoric mixes populist criticism of immigration policy, Wall Street behavior, and other countries' trade policies, while also citing support for business-friendly efforts such as lower taxation. The lack of detail from Trump about his policies and how he would implement them is a particular worry for investors.

"Trump has been light on policy substance so it's very difficult for the markets to handicap," said Dave Lafferty, chief market strategist at Natixis Global Asset Management, which manages $870.3 billion in assets. He expects market volatility to rise if Trump extends his lead in Tuesday's elections.

Some investors are particularly concerned about Trump's nationalist rhetoric, saying it is potentially destructive to a global economy that is already struggling. If it reduces trade flows then it could also hamper U.S. and global growth and hurt U.S. company profits.