Earlier quoted context omitted.
"Hugely popular" is an overstatement. "Everyone" excluding "Trump and his far conservative base" lost the election. And part of Trump's message was that the U.S. shouldn't be the guarantor of the security of the rest of the world. That was a hugely popular proposition with his supporters.
I don't think "hugely popular" is an overstatement if the number is really nearly 70% in favor. http://www.independent.co.uk/news/world/americas/us-politics...
So, not 70% of his supporters, especially considering not all Republicans voted for him, and some Democrats did.
And the math is funny anyway. I haven't seen recent registration numbers, but last I looked, it was roughly 1/3 Democrat, 1/3 Republican, and 1/3 unaffiliated. It might be 40%, 30%, and 30%, not sure; the point is that I seriously doubt 70% of the people who voted for Trump support the Paris treaty, and certainly this article does not say that, though it tries very hard to imply it.