In a pre-primary bombshell, the city’s Board of Elections was pummeled with questions Monday about how 54,000 Democratic voters vanished from the rolls in Brooklyn.

The voter rolls on April 1 showed the borough had 853,687 registered Democrats who are considered “active” because they voted at least once in the last four years.

But in November, there were 917,508, or 63,558 more.

The board said the numbers changed because many once-active voters were moved to the inactive list.

But that list grew by only 9,154 voters — from 82,807 to 91,961 — leaving 54,404 Brooklyn voters missing.

Voters on the inactive list can still cast a ballot if they live in the city.

But the “missing” voters are out of luck — their names have been stricken from the records.

Mayor Bill de Blasio said he was “confused” by the discrepancy and would seek an explanation.

“I’ve asked already today for an analysis of this from the Board of Elections,” the mayor said.

Bertha Lewis, director of the Black Institute, called for an investigation.

“There needs to be a probe. We know that 300K-plus have been purged without explanation,” she said.

Brooklyn Councilman Brad Lander called the purge “weird” and “troubling.”

Board of Elections Executive Director Michael Ryan said the mystery is easily explained: Because of retirements and staff illness, the voting list was not properly maintained in Brooklyn for six to eight months.

When staffers caught up with the backlog, he said, they purged voters who should have been removed earlier last year.