Latinos in the Rio Grande Valley are less Democratic than Latinos in the rest of Texas: many Mexican-Americans in Brownsville, Laredo and McAllen are fourth or fifth generation Americans, who have deep roots in the United States. They're more "Tejano" than Mexican. Many of these Mexican-Americans work for the border patrol and have concerns about border security. While I doubt that they're less than 55% Democratic, this would have an impact on the estimated White vote in the Rio Grande Valley. On top of this, it's quite likely that Latino turnout and voter registration is shockingly low compared to the Latino VAP.
Obviously, this is difficult to prove without detailed local knowledge but I have family members who grew up in Matamoros. Although Brownsville is considered by many Americans to effectively be part of Mexico, many Mexicans consider Matamoros to effectively be part of the United States. Mexican-American culture in parts of the Rio Grande Valley is actually pretty "American". This cannot be said for, say, Houston or Dallas, where most Latinos are recent immigrants or the children of immigrants.
Even though I'm sure my calculations are way off based on what was mentioned earlier, I generally believe the same: Latinos in the Rio Grande Valley are more established and therefore are going to be less Democratic, while whites (for the same reason mentioned a couple of pages back on my observation that whites in heavily-black rural counties vote more Democratic once the black share of the population has reached 60% or so) are going to be more Democratic. The real trick here is determining what the overall share of the electorate Latinos make up in each county.
Also, I'm playing around with new color schemes for the "Who Won the White Vote?" map. This is one I'm digging at the moment:
Update: there's no way white voters gave Obama more than 30-40% of the vote in the Rio Grande Valley. There's a very, very strong correlation between the percentage of White voters in a precinct and McCain's percentage of the vote. This correlation holds even when Whites make up 5-10% of the VAP.
I can't fathom whites gravitating towards Obama in towns like McAllen, Brownsville and Laredo: these communities were segregated 40-50 years ago. Mexicans lived in one side of town and attended different schools. I imagine that remaining Whites recognize that the Democratic Party of desegregation and Mexican-American culture.
