That would be somebody who follows the principle of seeking "the greatest good for the greatest number".

Emphasis on principle because a lot of people follow political theories claiming to be lefties whilst not in fact following that principle (tankies would be a good example).

Democrats aren't lefties, they're not even center - "what's in it for me" is their principle, same as all the other Neoliberals.

The lesser evil isn't the greatest good for the greatest number, in fact it's going in the opposite direction, just not as fast as the greater evil.