Like in 1968, when a third party candidate split the Democrat vote and carried 5 states, allowing Nixon to win. It wasn't Republican votes that (Independent and former Dem)George Wallace siphoned off.
The only time I saw where it didn't work was when John Anderson ran. Even a moderate Republican wasn't enough to keep people from voting for Reagan. I think Anderson tapped off as many moderate Democrat votes as Republican votes. The Democrats were split bad and Anderson did little to the solidarity of the GOP behind Reagan.