Memes

The problem with that mentality is that, as a leftist, you are always capitulating on your demands to keep the status quo, and this makes your whole political base weaker in the process. This makes the right wing to grow bolder, making the disillusioned people from the left to then leave it to adhere to right wing rhetoric, as they are seem to be the outsiders and not the status quo. When the worst evil eventually wins, it pushes policies to make their base stronger, and then makes the voter base more ideologically to the right.

So in summary, when the left consistently adheres to capitulating for the current lesser evil, it sets itself to defeat in the long term, as it becomes the maintainer of the status quo instead the force of change.