The chairwoman of the National Mediation Board has sharply criticized the federal agency's proposal to change a decades-old election rule that would make it easier for airline and railway employees to unionize, exposing a sharp rift at the agency's three-person board.

In a letter sent to more than a dozen Republican senators Monday, NMB Chairwoman Elizabeth Dougherty said the process by which the proposal was drafted by her two colleagues is "flawed," and she questioned whether the agency had the authority to make such a rule change.

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