BOSTON — When President Biden announced a slate of nominees to serve as top federal prosecutors across the country, Republican senators approved them without controversy.

All except for one.

When it came to Rachael Rollins, the nominee for United States attorney for the district of Massachusetts, Republicans vowed to block her ascent, calling her “pro-criminal,” “radical” and “dangerous.” They used a procedural maneuver to delay the progress of her nomination, the first time such a step had been taken in nearly 30 years.