The National Labor Relations Board aims to release proposed changes to its rules for union elections sometime this spring, the agency said.

The NLRB announced the timing of its election rule proposals in a memo to an American Bar Association group, made public April 4. NLRB Chairman John Ring previously announced that the board would take a piecemeal approach to amending its election rules, but he didn’t give a time frame for that effort.

The agency has rarely used formal notice-and-comment rulemaking to establish rules, instead acting through decisions in individual cases. The announcement on the election proposal timing confirms...