What is still considered a rivalry, even though it's mostly one-sided?
I don't have any good examples that I know of, but I am very curious. To me though, a rivalry is a rivalry when two or more of people are closely competitive. But is it really a rivalry anymore when someone has more victories over the other? I don't think so.
According to the rest of the world, the US doesn't have a left party. Democrats are right wing and Republicans are extremist right wing. The left is completely unrepresented in our government. Both major parties lean conservative (from a global perspective) and care more about helping major businesses and the rich elite than actually representing the people.
That's why there's a whole movement centered around "no war but class war." The American people are not actually represented and are instead pitted against each other in this fake "red vs. blue" distraction so we don't actually go after our political leaders, or weed out the source of the money behind the scenes that dictate their actions.
Technically speaking, there's the Green Party and PSL on federal ballots for many states, but the electoral system is positioned so that there's no foreseeable way that a party other than the two main ones could realistically win. So for all intents and purposes, there's no federal left-wing option.