The short answer is that the Republican candidates for President are union-busters. Any of them would be happy to suspend workers' rights. Here is a sample.
Walker, the worst of the GOP bunch, wants to get rid of workers' rights entirely.
Trump has a mixed record. He needs to play nice with unions because of his businesses, but despises teachers' unions.
Jeb Bush has pledged to "take the power…away from the unions and bureaucrats and give it back to parents."
Bobby Jindal "has long blamed teachers unions for impeding his education policy efforts, slamming their "Herculean efforts" to stop the expansion of charter schools and voucher programs that draw resources away from public schools."
Chris Christie " touted his years-long battle with the state’s teachers unions, in which he stripped away their tenure protections and pensions."
Carly Fiorina "has blamed unions for pay disparities between men and women, though that disparity is much smaller among unionized workers."
Rand Paul "has called for ending collective bargaining for postal workers and implementing "right-to-work" nationwide."
Any of these candidates would be bad for labor, not just unions.