You can tell this is happening from the fact that all elections seem to end up being a 49/51 result. There's some strange Nash Equilibrium thing going on.

that's just run of the mill polarization in the country, and that the party with more people hasn't been inspired to mass move them around the country for 5-8 months one year to register to vote and tip every election

That doesn't require anything nefarious. The parties can and do change their platforms and candidates to adapt to the electorate. If a party starts finding a way to win more votes, the other party will adjust to win them back.

It's like how prices at different gas stations tend to be within a few percent of each other. It's not conspiracy, just competition.