Acid test : An Idiom

Upcoming election is going to be an "acid test" for the Republicans.
Acid test originally implies to the test where they extracted the purest part of the gold using Nitric acid. Nitric acid dissolves all the other metals and alloys leaving behind the pure gold.
In general, acid test means a precise test that filters the unwanted things and gives a pure, concise results and finally confirms its purity.