The share of Republicans and Republican-leaning independents who believe that President Joe Biden’s 2020 election win was not legitimate has ticked back up, according to a new CNN poll fielded throughout July. All told, 69% of Republicans and Republican-leaners say Biden’s win was not legitimate, up from 63% earlier this year and through last fall, even as there is no evidence of election fraud that would have altered the outcome of the contest.
Both parties are capable of the same things by definition, but there's no universal law that keeps them both the same amount of good or bad. If one party continues to push policies that are bad (for whatever criteria of "bad" you're using), it stacks up and affects their overall average position on things. This process is reversible but it won't stop itself by default.