How did At-Will Employment weaken protections of the US worker?

I was told that I became into practice to be able to fire people in a union such as the United Auto Workers, which is currently the most powerful union, and easier to terminate employment of non-union employees and managers. Also At-Will Employment allegedly supports the owners and investors.

How did At-Will Employment weaken protections of the US worker?
The Democrats and Republicans benefit from it.
Reply:To weaken the union especially the United Auto Workers. Unemployment used to be 2 years but lazy people messed it up for those that are honest. It became more powerful in the 1990s especially in California.