People who were documented immigrants at one time, and served in the US military during that period, can become undocumented later. Military service can lead to citizenship, but the process has a lot of barriers and many people slip through the cracks. It is embarrassing as a country that we deport war veterans because they missed some obtuse paperwork deadline.

YouGov does polls like this with email invitations. You fill out your demographics, and they send out invites as polls become available. It is mostly polling on brands, but a significant minority of the polls are on political topics.