The folks over at the Public Religion Research Institute did a survey experiment. Here are the 3 different questions wordings and how they affected support among Democrats, Republicans, and Americans as a whole:
In short, merely telling people that there are "requirements" for illegal immigrants to become citizens increases support for the path to citizenship. A majority of Republicans support the path to citizenship when described this way. Interestingly, when told more specifically what those requirements were, support increased a bit, especially among Republicans, but not as much as when told that there are unspecified "requirements."
This suggests how Rubio and others could sell immigration reform to skeptics within the party. It also suggests that the public is less concerned with what exactly illegal immigrants need to do to become citizens, and more with the general existence of some sort of quid pro quo embodied in "requirements."