Alec Baldwin just got a huge part of the prosecution's case thrown out ... a day before his manslaughter trial kicks off.

The judge ruled prosecutors cannot introduce evidence of Baldwin's role as a producer in the film ... something that was a critical part of the State's argument that Baldwin's alleged recklessness led to the death of cinematographer Halyna Hutchins.

Baldwin was an Executive Producer on the film, but there were other EPs as well who had much more of a hands-on role.

Prosecutors argued Baldwin shirked his duties as a producer by allowing lax safety practices on set that led to Hutchins' death.

The judge said, "I'm having real difficulty with the state's position that they want to show that as a producer, he didn't follow guidelines, and therefore, as an actor, Mr. Baldwin did all these things wrong resulting in the death of Halyna Hutchins because as a producer, he allowed this all to happen."

The judge made it clear -- the decision-making on the set among producers went far beyond Baldwin, and ruled "The probative value is not substantially outweighed by unfair prejudice and certainly confusion [for the jury]."

So now, prosecutors can only argue Baldwin was reckless as an actor by pointing the gun in the direction of Hutchins and that he didn't check the chamber before firing. The problem with that argument -- the armorer, Hannah Gutierrez-Reed, has already been found guilty of manslaughter because she was responsible for ensuring the gun didn't have live ammo.

This ruling could very well cripple the prosecution's case.