Many super-wealthy Americans who "earn" most (or all) of their income through investments do not like unions. Unions bargain for higher wages and better benefits for workers, and these things cut into profits. In turn, lower profits equals less investment income.
Make no mistake about it--it's in the best interest of wealthy investors that workers get paid as little as possible and have the fewest possible benefits. Indeed, that was the whole point behind slavery: Reduce your labor cost as much as possible, and get richer and richer and richer.
And so unions were formed to provide a balance of power between laborers and rich non-laborers (i.e., wealthy owners, wealthy executives, and wealthy shareholders). Union power peaked in the 1950s, 60s, and 70s, and, it just so happens that, during this time, the middle-class grew like never before or since.
When the Reagan Revolution began in 1981, union power began to decline and tax-cuts-for-the-rich became the dominant philosophy of the land. And so here we are today with stagnant or dropping middle-class wages and a shrinking middle-class. Meanwhile, the rich are getting richer, and often stashing their money in foreign bank accounts to evade their historically low tax rates.
The political right, and their right-wing millionaire & billionaire political sugar daddies, have demonized unions and argued that they're bad for workers. Middle and lower-income conservative voters--unaware of our nation's history--have accepted this argument and voted against their own economic interest. They have been brainwashed to despise the very organizations that would increase their income and benefits. Right-wing millionaires & billionaires are probably having a good laugh over this. "Wow!," they must be saying, "We actually convinced them that smaller paychecks for them, and more riches for us, are a good thing!!"
New Deal policymakers knew that unions were a good way to counterbalance the inevitable and huge income & wealth disparities that capitalism causes. They also knew that unions were a good way to keep government out of the employer-employee relationship, as much as possible. Instead of the government nit-picking, legislating, and taking sides on every employer-employee issue, they felt it was better that unions be guaranteed the power to negotiate with the owners, executives, and shareholders, i.e., let the parties involved work out their differences.
And so, New Deal policymakers created the Wagner Act to protect the ability of Americans to bargain collectively, i.e., unions. Today, we are straying from union and New Deal philosophies, in favor of free market fantasies and "job-creator" myths. And so your paycheck remains the same while super-wealthy Americans get richer and richer off your labor, and as they cloak this ballooning disparity with anesthetizing words like "freedom" and "liberty."