The HBO documentary Hacking Democracy shows how easy it is. The Voting Machines In The Feature Film 'HACKING DEMOCRACY' Counted America's Votes In The November 2016 Elections.
During the 2004 presidential election in Florida, Gore had negative votes (or not enough positive ones to reach zero) on some of the voting machines. The smart card prior to being used is initialized and a summation of all the candidates votes is taken and checked to be zero. If some candidates have a positive number and others have negative values and the sum is zero, it passes its integrity check.