The New York Democrats basically checked out for this election.

Had they not, the Democrats may have retained control.

They did. It's called opposition research. Every well run campaign or major party does it. They knew.

The party that opposed him ran a literal dead person for a house seat in another state, and the dead guy won. It wasn't even unusual - several dead people have won elections. I'm pretty sure I saw a poll that showed that most voters didn't know he was dead. I doubt it would have affected him much if his fraud were exposed.

> Surprised his opponent or his party didn't do some vetting.

Implicit in your statement is that oppo research is at least as good as the journalism machinery. Perhaps the answer to your question is that they did some vetting, but are nowhere near as effective as the press. It's fashionable to dunk on the press (or "mainstream media") lately, but perhaps they have a useful function whose effectiveness does not wax and wain with the election seasons.