The “Nunes memo” alleges that a FISA warrant for a Trump campaign official was improperly obtained because the evidence it was based on came from a document paid for by Democrats. This contention betrays a misunderstanding of the way law enforcement works.
The judge issuing a FISA warrant must consider all the evidence and conclude that there is “probable cause,” which will be determined entirely by the credibility of the evidence. Its validity, not where it came from, is the key factor.
Here’s the point: If law enforcement officials do not consider all the evidence, regardless of source, they are not doing their duty. They would be suppressing evidence and would be guilty of malfeasance.
Contrary to the memo, the FBI actions in the case described were completely lawful and justified.