If a party can get your vote by being slightly less evil than the other party (and this applies to both the Republicans and Democrats to many voters), then they have no incentive to be good.
If you live in a society where parties are tending more and more evil, voting for less evil simply ensures that the trend will continue. Since “evil” in this case means “bad for ordinary people but great for me and people who bribe me”, there’s no reason for politicians enmeshed in the system, who rose under the system to do anything for the majority unless it benefits the rich and powerful more.
However politicians do sometimes change their votes or actions when in power based on needing to be elected.