It’s more that the parties basically agree on most things, that’s the centroid of public opinion the moderates live in. Each party then tries to figure out a set of scissors issues that divide the population, and enthuse supporters of those issues to turn out for elections. The more the centroid agrees on things, the more extreme the scissors statements have to be to peel voters out of apathy into action. Every time Congress does not pass a bill should be seen as bipartisan agreement not to change the status quo.