As a non American, I would say that the biggest reason for this ratchet effect is not the parties but the voters. Biden was by all metrics inching things along for the average population but they chose to vote Trump in again even though history would suggest that he would use the Presidency to enrich himself. Every time the GOP make things bad enough that the Dems get voted in, the Dems are held to blame for not being able to fix everything in 2 short years and the VOTERS usually take any advantage away by returning Congress or Senate to the GOP. And then the GOP not letting a single progressive action happen get off scot free and Dems are held to account. That is my summary of watching the US elections for 40+ years. It was a good democracy but American voters voting against their own best interest for decades have destroyed it.