Since 1972, every single presidential candidate who has won the popular vote has also won the Catholic vote. But with Catholics making up one in every four voters, pinning down what exactly the Catholic vote is becomes tricky.
Catholics no longer reliably vote for any one party, but historically, they have voted Democratic. In 1928, the first Catholic ran for president: Democrat Al Smith lost to Herbert Hoover. Dr. Robert Jones, CEO of the Public Religion Research Institute, told weekends on All Things Considered host Guy Raz that the link between Catholics and Democrats has to do with their position in the workforce.
"Part of it really is this alignment between the labor union movement and Catholics, who were really, up until the '70s, still really concentrated in Catholic enclaves in bigger cities; still working class ... in manufacturing jobs," he says.
A turning point came in 1972, when Republican Richard Nixon won the popular vote — and the Catholic vote — over Democrat George McGovern. Jones says the shift started gradually, with fewer Catholics voting Democrat in prior elections.
"So, we had begun to see kind of a slide. But it's not really until 1972 that we really see this division with Catholics really going Republican or Democrat depending on the election and looking really like the bellwether constituency that really goes with the general population," he says.
By the early '70s, Catholics had already been in the U.S. for generations.
"By the time we get to 1972, we have nearly a century of Catholic integration," Jones says, "with Catholics having upward mobility in terms of education, in terms of income and, I think importantly, moving out of Catholic enclaves in larger cities like New York and Chicago."
Now the mere size of the population indicates the unpredictability of its vote, Jones says.
"But I would say there are at least two key Catholic votes in the country, and they divide pretty cleanly by ethnicity," he says. "White, non-Hispanic Catholics, for example, in the last election supported John McCain over Barack Obama. However, if you look at the Latino Catholic vote, nearly three-quarters of the Latino Catholic vote supported President Barack Obama."
Obama received more Catholic votes in total, with the Latino vote putting him over the edge. In this election, both vice presidential candidates are Catholic. Further complicating their pitches is that many Catholics have different positions than the Church has.
Take contraception, Jones says, which the Catholic Church officially opposes even though the majority of Catholics don't have a problem with contraception, according to a Gallup poll. Another example is abortion. Catholic voters are divided on the issue, a 2009 Pew Forum report shows. The same is true for gay marriage, which the majority of Catholics support, according to a Public Religion Research Institute report.
President Obama is surging past Romney among Catholics. The latest Pew Poll gives him a 15-point lead among all Catholic voters. That's up from the two-point lead he had in June.
While the two candidates are virtually tied among white Catholics, Obama has a strong lead over Romney with both African-American and Hispanic voters overall.