The National Labor Relations Board’s already depleted field staff will drop by another 10 percent by the end of this fiscal year if Congress doesn’t intervene on the agency’s behalf, the union representing agency personnel said.

In a March 25 letter obtained by Bloomberg Law, the National Labor Relations Board Union told the top Democrat and Republican on the House Committee on Appropriations subcommittee that oversees the NLRB’s budget that the field staff has already plummeted from 1,136 workers to an estimated 857 between fiscal years 2011 to 2018. That represents an overall 25 percent decrease in field staffing.

Those...