The key to realignment is that Republicans are already a minority, and worse, there is a substantial minority of that minority (maybe 20%) who are sold out globalist - finance capital traitors.
Therefore.
Since the GOP is already a minority, and irretrievably split besides, the new party, in order to prevail and become the majority, will have to split the Democrat voter base to replace the 20% of the GOP who will leave, and ultimately to improve on those numbers.
You are forgetting about the independents and those the never vote ( 50+ million ).