You're making the mistake (which many others have made) that immigration is the reason Hispanics won't vote Republican; it's not.
Again, Romney's 27% is near the Republican's average among Hispanics since 1980. That means he did just as well as other Republicans who did not promote self-deportation did. Therefore, even if Romney supported a pathway to citizenship or amnesty from the start, there is no evidence suggesting he would have done better.
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It's not immigration!!!!
Oh, I know that. There is a reason Democratic performance among Hispanics has been consistently 60%+ since as long as they have been polled/monitored. Education, healthcare and other policies play big factors in guaranteeing Democrats that 60% - 65%.
However, as I was trying to say, Trump has made deporting 11 million people and building a wall the main premise of his campaign. That says a lot, and the perception among a lot of Hispanics is a full-on attack on their community. I live in FL and I do live near quite a lot of Hispanics/interact with them very often. The perception here is absolutely
toxic for the reasons I specified. He's highly offensive and rude. There are just so many problems with him and his campaign that justify his 80%+ unfavorable rating among Hispanics.
It's also been said, somewhat correctly I think, that Republicans can maintain a stricter immigration policy and still get Hispanic votes, such as in Texas. The key is not going at that voting bloc with machine guns blazing while calling them rapists and threatening to deport 11 million people (aka, a lot of people they know, or even their family members).
I don't get why this isn't obvious.