For the first time since the days of poll taxes and literacy tests a half-century ago, the Justice Department will be sharply restricted in how it can deploy some of its most powerful weapons to deter voter intimidation in the presidential election.

Because of a Supreme Court ruling three years ago, the department will send special election observers inside polling places in parts of only four states on Election Day, a significant drop from 2012, when it sent observers to jurisdictions in 13 states.

And in a departure from a decades-old practice, observers will be sent to only one state in the South, where a history of discriminatory voting practices once made six states subject to special federal scrutiny.

The pullback worries civil rights advocates, who say that Donald J. Trump’s call for his supporters to monitor a “rigged” electoral system could lead to intimidation of minority voters at polling places.