Montana is the only state that does not have “at-will” workers. I have advocated for Maryland, by legislation, to join Montana and as your Senator will push a federal end to at-will employment. The courts adopted a slave-master treatise in 1877 (ironically a century before I was born). Workers lack due process and this must come to past!
States and the District of Columbia have exceptions to at-will such as: public policy (obvious discriminatory reasons that you can’t be fired), implied covenant of good faith and implied contract.