Democratic presidential candidate Kamala Harris campaigns in Michigan while her Republican rival Donald Trump will stop in three eastern battleground states in the final days of an election that could be decided by a small slice of American voters, Reuters reports.

Opinion polls show a historically close race, and both campaigns say they are not sure who will prevail in the seven states that are likely to determine the outcome on Tuesday.

A poll showing Harris leading in Iowa — a state Trump won easily in the past two elections — raised the possibility of an unexpected outcome, though another poll showed her trailing in that state.

Harris is due to campaign in East Lansing, Michigan, a college town in an industrial state that is viewed as a must-win for the Democrat.