If I were a Latino -- and one determined to vote on stereotypically "ethnic" issues like immigration -- there's no competition: I'd vote for John McCain, even if I actually liked some of Barack's positions a little more.
Why? Because McCain has actually taken the heat in order to espouse stereoptypically pro-Latino positions. What's more, he's the most stereotypically pro-Latino candidate Republicans could have nominated.
If notwithstanding that fact, Latinos flock in great numbers to support Barack, a lot of Republicans will figure that, if even John McCain can't win Latino support, there's no way any other GOP candidate can. As a result, Latinos will find themselves in the uncomfortable bind that too often confronts African-Americans -- taken for granted by the Democrats because Dems believe that Latinos will simply refuse to support a Republican. And Republicans will sadly conclude that they must look to other groups in order to build a winning coalition, with little political incentive to pay attention to stereotypical Latino concerns.
If Latinos support McCain, on the other hand, it keeps them viable "players" in both parties. Strikes me as an easy strategic choice.