A survey by polling firm Opinium showed the campaign for Britain to leave the EU holds a one-point lead over the “In” camp ahead of Thursday’s referendum.
Though polls are extremely close, bookmakers had been pointing to betting patterns in favor of the U.K. remaining in the EU.
Adding to the cautious tone of the day, oil prices fell about 1 percent after a smaller-than-expected weekly U.S. inventory draw.
Investors are also keeping an eye on U.S. Federal Reserve Chair Janet Yellen’s second day of testimony to Congress.
On Tuesday, Yellen played down the risk of a recession, but warned that the British referendum and a U.S. hiring slowdown posed risks to the economic outlook.
Yellen’s comments pointed to a cautious central bank, virtually ruling out a July rate hike.
International Monetary Fund Managing Director Christine Lagarde also said on Wednesday that the Fed should avoid “abrupt” interest rate moves and focus on stability.
Trading volume remains thin as investors adopt a wait-and-watch approach.
“Today is a whole lot of nothing,” said Art Hogan, chief market strategist at Wunderlich Securities in New York.
“We are clawing back some of the losses from last week and are in a bit of a holding pattern ahead of tomorrow’s vote. I think if there is a ‘Leave’ vote, then a July rate hike is definitely off the table and we might be looking at just one hike in December.”
At 12:48 p.m. ET (1648 GMT), the Dow Jones industrial average .DJI was down 24.21 points, or 0.14 percent, at 17,805.52, the S&P 500 .SPX was up 0.09 points, or 0 percent, at 2,088.99 and the Nasdaq Composite .IXIC was down 0.57 points, or 0.01 percent, at 4,843.20.
McDonald’s (MCD.N) 1.8 percent fall weighed the most on the Dow.
Six of the 10 major S&P sectors were higher, with the health index .SPXHC up 0.56 percent after a report said Medicare spending levels did not exceed targets and did not warrant the formation of a cost-control board.
Tesla Motors (TSLA.O) was down 9.2 percent at $199.45 after the Elon Musk-owned electric car maker made an offer to buy his solar installation firm SolarCity (SCTY.O) in a deal worth as much as $2.8 billion. SolarCity was up 5.4 percent at $22.35
Adobe Systems (ADBE.O) was down 5 percent at $94.73 after its second-quarter revenue and full-year revenue forecast just about met analysts’ estimates.
Advancing issues outnumbered decliners on the NYSE by 1,531 to 1,379. On the Nasdaq, 1,382 issues rose and 1,299 fell.
The S&P 500 index showed 25 new 52-week highs and 3 new low, while the Nasdaq recorded 50 new highs and 38 new lows