1. Immigration/Refugee Issues - This is clearly an issue he feels passionate about. He was the most pro-immigration Governor during his tenure. He talks about immigrants as New Americans. He speaks of their struggle to come to America with passion and treats them with dignity. I particularly like how he talks about immigration everywhere he goes not just pandering to Latino audiences. When the children of Central America were pouring over our borders seeking safety here, he stood up to the White House and Hillary and said we should not send them back to certain death. It goes against everything we stand for as Americans. And he backed that up with action. He worked with social service groups and religious organizations to take in more child refugees than any other state per capita and put them in family fostercare not warehouses. He also passed the DREAM Act for Maryland, allowed undocumented Americans to get Driver's Licenses, etc.
2. Environment - He worked to turn Maryland into a greener and safer state through pushing through the largest wind farm off the eastern shore, making regulations so tough for fracking that it put a de facto ban on it in Maryland, etc.
3. He has a laundry list of progressive accomplishments that he is proud to run on. He has been a forceful advocate for Obama and his accomplishments.