Just because someone engages in protected conduct doesn't mean you can't fire them. It just means you better have your ducks in a row when you do so.
Case in point: the saga of Nicole Oeuvray and the Art Directors Guild. Oeuvray, who served as the guild's accountant for 16 years, had been one of leaders of a campaign to organize the guild's employees into a labor union.
The guild fired Oeuvray for what it called "significant performance issues" — she bounced a guild check, produced late financial reports, and had issues working well with others.
Oeuvray, however, argued that the guild fired her because of her union activities. In support of her claim, she relied on the anti-union bias of her boss, guild Executive Director Chuck Parker, including his profane outburst against the union.
In a unanimous decision, the NLRB held that the guild lawfully terminated Oeuvray because of her performance issues. "The reasons listed in the termination notice concerned performance matters that preceded Oeuvray's union activities, and Oeuvray admitted to missing yet another financial reporting deadline just prior to her termination." Thus, her union-supporting activities were not a "a substantial or motivating factor in the adverse employment action" (the legal standard in play here).
The lesson here is "Document, Document, Document!" There are few terminations that can survive scrutiny without proper documentation. Your odds as an employer go down exponentially if you pair a lack of documentation with a termination of and employee who has engaged in protected activity. As this case illustrates, a poor performer is a poor performer, regardless of union organizing activity (or any other protected conduct). Without a legitimate paper trail, however, you will find it very difficult, if not impossible, to do anything about it.