A tight U.S. presidential race is leading some investors to brace for an unclear or contested election result that could trip up this year’s booming stock market rally.

With less than a month before the election, polls and prediction markets show Democrat Kamala Harris and Republican Donald Trump in a virtual dead heat.

Harris led Trump by a marginal 46% to 43% in a Reuters/Ipsos poll released on Tuesday, a tighter race than the same poll showed a couple weeks earlier.