Neither Trump nor republicans want the affidavit released, just as they didn’t want the search warrant released.
This is a very common republican tactic: by demanding to have something released when they believe it is unlikely that will happen, they can raise doubt while at the same time pretending that they have nothing to hide.
Well, that backfired when it came to the search warrant: Garland got the warrant and receipts released, to the vast embarrassment of Trump and the GOP.
Now they’re screaming for the affidavit, as if somehow that will fully exonerate Trump. This time, they feel more confident that it will not be released, as releasing affidavits is very rare.
Of course, they might actually get their wish—and if they do, they will surely regret it. An affidavit would show the cause the FBI had in asking for the search warrant—and that could potentially be worse for Trump than the search warrant was.
Of course, republicans have another motive: the affidavit would reveal the FBI’s source(s) that they supposedly have within the Trump inner circle, or at least clues as to who it could be. This would help them because it would give them a target to endlessly smear and accuse—not to mention signal to their fringe crazies that open season is on for snitches.
If the affidavit is released, which it probably won’t be, it would hopefully be heavily redacted to hide all sensitive information.