A phrase to mean "if that point is solved, the rest of the problem will be solved"

I am looking for a phrase that is in the same spirit such as "single point of failure", or "bottleneck". Example:

Being two engineers short is <-term goes here-> in this situation. If we find two engineers, the project will meet the deadline.

Another one:

Electric cars are improved enough to be on par with internal combustion engine cars. However, the long charging time is <-term goes here-> behind the problem that electric cars are not as widespread as internal combustion engine cars.

Solution 1:

crux, Oxford English Dictionary

The chief problem; the central or decisive point of interest.

Example: 1934 R. Benedict Patterns of Culture vii. 232 The crux of the matter is that the behaviour under consideration must pass through the needle's eye of social acceptance.

The OP's two examples would then be slightly rewritten to read:

Being two engineers short is the crux of our problem. If we find two engineers, the project will meet the deadline.

Electric cars are improved enough to be on par with internal combustion engine cars. However, the long charging time is the crux of the problem causing the slow inroads electric cars are making on internal combustion engine cars.

In rock climbing, the crux is, according to Wikipedia, Glossary of Climbing Terms