The Republicans pass a new tax reform law that benefits wealthly Americans. Shortly thereafter the economy takes a nose dive. The Democrats claim that the the tax reform caused the economic woes and they push to get rid of it.

This is post Hoc fallacy

Explanation:

It is a False assumption that because one event occurred before another event, the first event must have caused the second event.

The Republican passed a new tax reform law, let's say this is the first event. Thereafter the economy went on a nosedive. This is the second event which the Democrats believe was caused by the Republican new tax reform law.