This is really good for Democrats. Arpaio was basically Trump before Trump ran for office. He is an arch-conservative and anti-immigration activist. He is almost certainly less electable than his opponent. (he lost last time in the general election), so if he wins Dems are in good shape. If he loses, especially when it is this close, many of his supporters would likely reject the results, especially given the number of mail-in ballots, which Trump has claimed lead to fraud. So an Arpaio win or a close result is good for Dems.