Why did most northerners support tariffs and most southerners opposed him?

The Northerners supported tariffs because if the South stopped buying from Europe, then the North would make more profit from the South buying from it. Southerners opposed this idea because if they stopped buying from Europe then they would not be able to trade. 

(I'm not smart, sorry if this explanation sucks. What are you learning about?)

He wrote it on behalf of the united states I'm pretty sure.

This is what I found...Open Door policy, statement of principles initiated by the United States in 1899 and 1900 for the protection of equal privileges among countries trading with China...Does this help you?