There has been disagreement as to the levels of national Latino support enjoyed by both President Bush and Senator Kerry in the 2004 Presidential elections. The National Election Poll (NEP) and Los Angles Times exit polls indicated that 44% and 45% of the Latino electorate respectively voted for President Bush on election-day. However, sample design flaws have called those findings into question.
The William C. Velásquez Institute (WCVI), the non-profit, non-partisan sister organization of the Southwest Voter Registration and Educational Project (SVREP), founded by Willie Velasquez in 1985 conducted the first national exit poll of Latinos on November 2, 2004. Never before in the history of presidential exit polls had an organization conducted a poll exclusively of Latino voters. The findings of this historical poll contradicted those of the NEP and Los Angles Times. The WCVI poll found that only 33.0% of Latinos queried on election-day voted for President Bush while 65.4% supported Senator Kerry. Another 1.4% of Latinos polled indicated that they had cast their votes for Ralph Nader and his vice-presidential running mate.
What was most important about the WCVI findings was not that they differed greatly from those of the NEP and LA Times polls, but that they only varied slightly from five other pre-election polls. Additionally, these findings indicate that Latinos had changed their voting patterns little since the 2000 general election. In 2000 both the WCVI and an independent research source found that President Bush had received only 35% of the Latino national vote. When factoring in the margin of error for both polls the 2004 findings of 33.0% support for President Bush indicate that Latinos had continued to vote in the same manner as they had historically.
The most important question underlying the disagreement between the findings of the NEP, Los Angeles Times and WCVI polls is why the findings of Latino support for the presidential candidates differed so markedly? The twelve to thirteen point variance goes well beyond any possible margin of error. The answer may be found in the methodologies utilized in designing the sampling frame. In other words, if one compares the samples of all three polls one will find why a difference exists between the polls’ respective outcomes.
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http://www.wcvi.org/latino_voter_research/polls/national/2004/flores.html#_ftnref1