Voters in Mexico are participating in the country’s largest election ever — casting votes Sunday to fill more than 20,000 local, state and federal positions and almost certainly elect their first female president.

But rampant violence has marred the road toward one of the most consequential elections in Mexico’s history.

Criminal groups have taken over large parts of Mexico as they fight for territory to traffic drugs into the U.S., make money from migrant smuggling, and extort residents…