because you're not a single-issue voter, and you agree with the majority of their policies and are actually trying to pull them in a less-extreme direction?
Seriously, your argument is just the party-level version of "why would you run for office in america, a country that elected someone who denies climate change and says vaccines cause autism?" Easy - to lead them in a positive direction!
Seriously, your argument is just the party-level version of "why would you run for office in america, a country that elected someone who denies climate change and says vaccines cause autism?" Easy - to lead them in a positive direction!