The government may be forced to intervene in the strike, as it has done with recent port strikes, given the importance of the postal carrier to the economy.
Despite Republican hostility, and even with control of both chambers of Congress and the White House, cutting federal transit funding may prove difficult.
The group consists of 40 companies, and was originally founded by e-commerce giant Amazon, outdoor recreation retailer Patagonia, and German coffee chain Tchibo.
Large employers including Amazon, Starbucks, Trader Joe’s and REI have responded to increasing attempts to unionize by holding captive audience meetings.