NEW YORK — Wall Street is breathlessly on edge over the United Kingdom's Brexit vote on Thursday, with many analysts fearing a major sell-off and possible global slowdown if the country votes to leave the European Union.

Many Democrats also fear a vote by the U.K. to leave the EU will suggest that a nationalistic, anti-immigration political wave that is also sweeping the United States could be much stronger than expected.

Current polls suggest the "Remain" side has a narrow lead in the U.K. But the referendum could swing either way and Wall Street analysts fear that a surprise "Leave" victory would send stock prices sharply lower, increase uncertainty about a complete unraveling of the EU and damage a United States economy that is barely growing and remains highly susceptible to outside shocks.

"My expectation is Remain wins," said Jim Paulsen, chief investment strategist at Wells Capital Management. "If Leave wins there will be an immediate reaction with sharp losses in stock markets across the globe. Emerging market stocks will also be hit hard. Oil prices will decline significantly. The U.K. pound will decline significantly."

The shock to the U.S. could be even greater because for months, polls and betting markets alike suggested that Remain would win fairly handily following stark warnings from leaders across the globe — including President Barack Obama — that a vote to leave the EU would be terrible for both the British and the global economies.

"An exit vote would catch investors flatfooted, as most of us are relying on local betting figures to conclude that Brexit is an unlikelihood," said Jack Ablin, chief investment officer at BMO Private Bank. "Banks with London financial centers would bear most of the brunt of selling, as investors anticipate trading would shift to a euro-based city, like Frankfurt. We would expect a dollar and yen rally against the pound and euro weakness as capital shifts away from the region. It would be hard to predict how hard stocks could get hit since the blowback from the vote would play out over a couple of years."