Most of their assumption is from data gathered from the Regan era. To this date, Regan was the only modern-day president to get any kind of immigration reform done, I believe they legalized over 2.7M people. This leads to a massive surge to the border and many of the GOP credit the 11M illegal immigrants to that act.

Because of that, any kind of immigration bill in the GOP's eyes needs border security since it will lead to a massive influx of people to the border to get citizenship. Think of what's happening right now at the border times 1000. Those are the assumptions being made.