In some jurisdictions --- I don't know how common this is, but it's the case in mine --- uniformed police officers are prohibited from the polling place unless

In some jurisdictions --- I don't know how common this is, but it's the case in mine --- uniformed police officers are prohibited from the polling place unless requested by election judges because of the potential that it will create a voter intimidation situation (keep in mind that these law enforcement officers are under the command of one of the people on the ballot). So I would conjecture that some areas may ask police to appear in plainclothes, although here police are simply not present at all unless requested---the county did hire a private security firm to keep an eye on some locations though due to problems during early voting. This is basically because the right to eject someone from the polling place rests with the election judge, not police or security, but they're temporary employees and not trained in deescalation.