The private sector makes up 80% of all American employment. 40.7% of the public sector workforce is covered by union representation, in contrast to only 7.6% of the private sector. Thus, to the extent that union-represented public employees enjoy compensation packages and working conditions unavailable to most of the general public, public employee unions will become less sympathetic. Recent results in Wisconsin, San Diego, and San Jose confirm this lack of sympathy.
But in another sense, the gap between private and public sector unionization rates strengthens the cause of public employee unions, because it makes it impossible for a politician to be a friend of labor and critic of public sector unionization.
Unlike, for example, President Franklin Roosevelt. In 1937, FDR famously wrote to the President of the National Federation of Federal Employees that "meticulous attention...to the special relationships and obligations of public servants to the public itself and to the Government" leads to the conclusion that "the process of collective bargaining, as usually understood, cannot be transplanted into the public service." His reasoning was sound, and still relevant today:
[Public service] has its distinct and insurmountable limitations when applied to public personnel management. The very nature and purposes of Government make it impossible for administrative officials to represent fully or to bind the employer in mutual discussions with Government employee organizations. The employer is the whole people, who speak by means of laws enacted by their representatives in Congress. Accordingly, administrative officials and employees alike are governed and guided, and in many instances restricted, by laws which establish policies, procedures, or rules in personnel matters.
And yet, easy for him to say: he had just signed the landmark Wagner Act into law two years prior. FDR ran no risk whatsoever of being called anti-labor. Is it conceivable that FDR would have said this today? Would FDR, one of the canniest politicians in American history, have declined to avail himself of the tremendous political support that public employee unions can now provide? Dubious.
FDR explains why collective bargaining makes sense in the private sector but not in the public. But however theoretically-possible this position may be, in our own time, it's politically impossible. If you're opposed to policies cherished by public employee unions, then you're inevitably seen as anti-labor. This is because the cause of private sector unionization is too far gone, and the cause of public sector unionization too far advanced.