Michael O'Brien argues it's a myth:
The religious assemblage, which has evolved over the past century from a strong Democratic constituency into a national election bellwether, is no longer discernible from most other voter groups. As the community has become less homogenous and more assimilated into mainstream culture, so has its voting habits – sending many politicians on a fool’s errand in pursuit of the “Catholic vote."
The hierarchy – and the theocons – have effectively put their collective weight behind the GOP, as the above ad (from a theocon group) reveals. For them, contraception, abortion and gays are "non-negotiable" and you risk eternal damnation if you vote for Obama. No other issues – such as the Ryan budget or the repeal of universal healthcare insurance – matter to the same extent. One wonders whether this Christianist campaign will do much more, though, than mobilize the hard right, who were already anti-Obama. But the hierarchy's effective endorsement of Romney might swing a few votes, as Douthat notes:
The Catholic vote does look a lot like the American vote in microcosm, encompassing liberals and conservatives, the lukewarm and the devout, the partisan and the uncommitted and everything in between. But as National Review’s Ramesh Ponnuru points out, there is evidence that Catholic swing voters are slightly more up for grabs than the average independent. Ponnuru notes that George W. Bush “improved his share of Catholic voters between 2000 and 2004 more than he did his overall share; and the Republican share of the Catholic vote fell a bit more between 2004 and 2008 than did the Republican share of the overall vote.”