To comment on this.... Unionization of a workplace is not to be a "political" issue but an individual private work issue.
The concern with this change in voting is simple...
The Railway Labor Act - the Federal Law that governs these things - does not allow the NMB to change the wording or the intent of that Law.
The RLA states that the majority of the "workforce" must vote for union representation in order for that union to come in.
It does NOT say that the majority of those voting.
The intent is to get those FOR the union to vote YES.... NOT for ALL workers to vote.
READ the RLA. It is very clear. Also, the RLA forbids anyone from serving on the NMB if they have an interest in seeing unions come into a company.
IF a workforce wants a union then the majority of those people WILL vote FOR a union.
This is not a politcal issue as you made a parallel with voting for local and fed. govt. issues.
Again, read the RLA - That piece of legislation is the governing article for this issue.