Past performance estimates future results, and it shows that while Democrats don't actively make things worse, they also don't actively make things better. This is illustrated in the "ratchet and pawl model.
When the Republicans are in power they move things rightwards. When the Democrats are in power they don't move things in either direction. The net effect is a move to the right, and you cannot influence that by voting, and the overall rate of movement depends on how often each party wins, which you can influence by voting.
The chance that a third party wins and moves things leftwards is zero.
When the Republicans are in power they move things rightwards. When the Democrats are in power they don't move things in either direction. The net effect is a move to the right, and you cannot influence that by voting, and the overall rate of movement depends on how often each party wins, which you can influence by voting.
The chance that a third party wins and moves things leftwards is zero.