The Democrats and Republicans mostly despise each other, but the establishment from each naturally have a few things in common. I am not a centrist "both sides" guy. However, with rare exceptions, members of both major parties are pro-capitalism, just for starters.

They are not secretly working together, but they do have somewhat overlapping goals. And some of those goals do screw over working people. Private health insurance tied to your employer is something both parties praise and support. That's one easy example.

And some of them cross the aisle and openly work together on bipartisan issues.

Finally, the "left" in the United States is really not left. Both parties pull rightward. The Republicans pull rightward much harder than the Democrats. A few progressives and democratic socialists try to pull us left.