See his entire 8-point argument here. Following are just a few of them.
3) One of these candidates, Trump, denies the existence of global warming, calls for increasing use of fossil fuels, dismantling of environmental regulations and refuses assistance to India and other developing nations as called for in the Paris agreement, the combination of which could, in four years, take us to a catastrophic tipping point. Trump has also pledged to deport 11 million Mexican immigrants, offered to provide for the defense of supporters who have assaulted African American protestors at his rallies, stated his “openness to using nuclear weapons”, supports a ban on Muslims entering the U.S. and regards “the police in this country as absolutely mistreated and misunderstood” while having “done an unbelievable job of keeping law and order.” Trump has also pledged to increase military spending while cutting taxes on the rich, hence shredding what remains of the social welfare “safety net” despite pretenses.
8) Conclusion: by dismissing a “lesser evil” electoral logic and thereby increasing the potential for Clinton’s defeat the left will undermine what should be at the core of what it claims to be attempting to achieve.