Check your contract, job description, and company P&Ps (policies and procedures). In theory you were shown these when you were hired, and by accepting the job you agreed to abide by them. So, do either of these specifically state that you must meet the 80% accuracy rating within that time frame? If so, you're kinda out of luck. If not, then you may have some rights here. IF they specifically say something else, then you definitely have some rights here.
You could force the issue, but why? Do you really want to force a company to keep you on? Think about how they'll treat you. It may be time to just move on, and consider this a learning experience... make sure you have all this out in the open, up-front and in writing before accepting a position.
However if you want to keep this job, then you should be able to determine if you have some legal ground.
Good luck.
Edit: Contract law may not hold up against employment law. If your state is a "right to hire" state (AKA "right to fire" state) then even contracts and P&Ps and job descriptions won't help. Employees pretty much have no rights.
If you are a contractor, then they must abide by the terms of your contract.