Simple:
Step 1: Identify the crime.
Step 2: Have probable cause - aka evidence - that Trump committed it;
Step 3: Subpoena the further evidence that is relevant to the crime.
Skipping Steps 1 and 2 may be what DU-grade Proggies want to do for their fishing expedition, but that is not the things work in the US. DU-grade Proggies don't even have Step 1, a crime to be investigated.