https://en.wikipedia.org/wiki/New_Deal#Historiography_and_ev...
Most paint it as a failure of capitalism, the rest as a direct result of government manipulation of the banking system, trade disruption caused by Smoot-Hawley, and huge tax increases.
Depending on who you ask, the New Deal was either ineffective because it was fundamentally flawed, or ineffective because it wasn't big enough and didn't raise enough debt. [2]
Basically, "economic science" as usual...
It seems unlikely that some huge portion (say, > 90%) of those historians further believe that New Deal policies had no or little positive effect on the Great Depression. Are the "credible historians" just the ones who happen to agree with you on this topic?
So yes, I believe that most of those 51% and 74% believe the effect was nonexistant or small. Typically, something like "if FDR had spent as much on the New Deal as he did during the War, it would have ended the Depression". [1] (I.e., it was too little, as in my previous comment.)
I welcome any references to historians claiming the New Deal largely fixed the Great Depression. (Or really, any option from a historian more positive than the quoted article.)
Or...you know, just silently downvote this comment :)
[1] https://www.thebalance.com/the-great-depression-of-1929-3306...
Absolute numbers are important, but the rate of change more so. Were unemployment raising to 25% or already going down? It's the difference between those 19% being a meager improvement and staving off absolute disaster.
We actually have a pretty good idea of what worked and what didn't during the Great Depression and mainstream economic historians[1] seem to have reached a rough consensus.
In school, though, we teach the Great Depression through the lens of political history. So for that purpose it doesn't matter what was really causing it, just what people at the time thought was causing it.
[1] I'm excluding Marxists, Austrians, MMTers, etc here.
https://en.wikipedia.org/wiki/Great_Depression#Mainstream_ex...
EDIT: A while ago Krugman had a very good piece trying to explain how deflation can cause problems. Different sorts of economists often disagree about whether its interest rates or aggregate quantities or expectation traps or whatever that's the most important aspect of tight or loose money but they all agree that these things are real and meaningful and generally explain depressions. http://www.slate.com/articles/business/the_dismal_science/19...