Why has union support in the united states gone through cycles of resistance to strong support?

Because according to the time and posture, Labor Unions have benefited the workers or act against them. Or sometimes they acted looking for god but actually the impact wasn't not that good.

Explanation:

First of all, Unions are established as a mechanism of coordination to establish objectives and provide it's members tools to achieve them. However, there have been times in which for example the unions helped the workers get better conditions, salaries benefits and perform better, one of these times is the 1920s and 1930s. Nevertheless, very lately unions have turned into a debate because their leaders have agreed on terms with owners or management to keep the salary under the 15 dollar mark. Also, because the leaders don’t really push management to get better conditions and benefits but ask for the least they can get and in many cases, union leaders get wealth by unexplainable reasons outside the legal mark of rewards. In other cases like UPS, drivers asked for better wages and benefits, however, the response was a punishment of firing without valid arguments, and the union didn't support them, they retire. But there have been times in which the impact is positive as in the work labor unions that managed to get better salaries to their workers instead of what the law established.

the awenser wuold be slaves