I was curious about who the 64% of Americans are, because I've never done a poll in my life. This is what I found:

NORC gathers data for the polls through its probability-based AmeriSpeak® Panel, a breakthrough survey approach that achieves an industry-leading response rate and includes difficult-to-reach demographic groups, such as rural and low-income households.

For AmeriSpeak's Omnibus survey, 1,000 nationally representative adults age 18 and older are drawn from the AmeriSpeak Panel and are interviewed online and by phone.

The people that are drawn from the panel are all preregistered. So these people sign up, and are asked questions every month. The same people. Every month. Then they do some sort of probability math on those numbers and run it as a news story.