Congress is an interesting place to say the least! We do not have near the protections that are standard for employment for instance there is not need for cause to be fired. Your boss could just walk in one day and decide ya know nope you are done and well that's it!
Now I do not hear about that being an issue very often but I have heard of it before. For those that work in other parts of the Federal Government typically once you hit I want to say 7 or 8 years and have continued to move up and perform well of course you become practically unfireable but for Congress, it is a different animal!
From what I remember read a couple of years ago Congressional Employees have the fewest protections of any sort of employment opportunity. Our HR for instance is often the same boss would who fire people sooooo we don't really have HR but if you work with good people and like I do the only uncertainty faced is in situations where the House could flip. On the Senate side, this isn't an issue because they divide the money evenly between the two parties but not in the House its a 70/30 split.