The pollster had been releasing surveys since June, shifting from one midterm battleground to another: Michigan. Maine. Kansas.

On Tuesday night, after the polling website FiftyPlusOne contacted the pollster, Wedgewood Polls, with questions about the authenticity of its surveys, the team behind the supposed data firm acknowledged that almost all of its work was fake. The whole thing, Wedgewood claimed, was an “experimental project.”

The fake polls were highlighted by well-known polling aggregators, including on the pages The New York Times uses to track surveys published by outside pollsters in key midterm races.…