You will probably hear that Rubio has nothing to fear over supporting immigration reform. They will argue there are others in the race (Jindal, Christie and Bush) who have supported immigration reform.
However all of the other people are outside Congress. They never have to vote for the bill and can quickly find the worst part of the bill and say “I’d never support that part of the bill, which Marco Rubio helped push through.” So they can cherry pick it to make sure everyone knows Rubio helped make the bad stuff a reality.
Second, Rubio is Latino. Any racial element to opposition to immigration reform will be taken out on him as the most prominent Latino politician in the country.
Rubio has to figure out a way to change the subject quickly.