At some point, a saturation point is reached, and then a tune-out point is reached.
When I'm in the States, and I see an ad for ANYTHING come on too often during the course of a movie or an evening on the same channel, at some point I get sick and tired of seeing it. Beyond that, if it still comes on, I get so annoyed that I make a mental note to NOT ever buy that product or use that service, especially if the ad is irritating.
I'm sure most of the Republican ads were already a little insulting to the intelligence of many viewers, even if they agreed with the ads at the start. But when people start to scream "enough already" at their TV screens, then the ad has outlived its usefulness and begun to have the opposite effect of what was intended. Only the TV station gets rich (along with Rove, if he got a percentage of the purchase price kicked back to him as a fee).