Sen. Hillary Rodham Clinton and her advisers have referred to today's Texas and Ohio primaries as her campaign's "firewall" against Sen. Barack Obama's surging popularity. In both states, the New York senator's barrier is built on the same foundation — the Catholic vote, and that fact has intriguing implications well beyond the primaries.

Today, one in four Americans is a Roman Catholic. What makes all this particularly significant in this political year is that exit polls across the country show that Catholics are casting 25 percent of all the primary votes — mirroring, in other words, their percentage of the general population.

The importance of that turnout is amplified by the fact that Catholics now constitute the only true "swing vote" among discrete electoral groups followed by pollsters and social scientists. Though they no longer march in quite so precise a lock step, evangelicals still tend overwhelmingly to vote for the GOP candidate. Blacks and Hispanics remain reliably Democratic, as do nine out of 10 Jews.

In the mechanism of American politics, Catholics are the pendulum — and they consistently swing toward the winner. Catholics have gone for the winner of the popular vote in nine consecutive presidential elections, which means that, since 1972, they've supported five Republican candidates and four Democratic ones.

Through Super Tuesday, Clinton appeared to have a 2-to-1 lock on the Catholic bloc. After Super Tuesday, the Catholic pendulum — like the campaign's momentum — swung decisively toward Obama. The most recent polls say Catholic sympathies are too close to call in Texas and Ohio.

All this is likely to matter well after the Democratic convention because there's every indication that the Catholic pendulum is likely to swing away from the Republicans in November. George W. Bush lost Catholics when he ran against Al Gore, but narrowly carried them against John Kerry, mainly on the strength of Karl Rove's and Deal Hudson's adroit use of the abortion and same-sex marriage issues in major Midwestern and mid-Atlantic states. (Hudson was then the White House's liaison to conservative Catholics.) The conventional wisdom following that election was that Catholics had begun to split along the same lines as other Americans, with those who went to Mass weekly voting Republican, the same way Protestants who regularly attend services already do. In the 2006 midterms, however, Catholics — particularly in the industrialized Midwest — swung away from the GOP and gave the Democrats a 10-point margin.

Why?

Unlike evangelicals, Catholics have a long tradition of theologically nuanced social thought on which to draw: the church's so-called social gospel. The Catholic bishops, for example, have been demanding national health insurance longer than Clinton has been alive, and the term "living wage" was introduced into the American conversation by Catholic commentators. Similarly, the church's support for unions and opposition to restrictive immigration policies are well-articulated. More recently, both the Vatican and the American bishops have opposed capital punishment and pre-emptive warfare.

Now, the typical Catholic probably doesn't know an encyclical from an insecticide, but this sort of social thinking and preoccupation with social solidarity and a "preferential option for the poor" are woven into the fabric of even casual parish life. It's this background that has tended to make Catholics swing voters: uncomfortable with the sort of litmus-test left that once kept Pennsylvania's pro-life Democratic governor, Bob Casey, from speaking at a national convention, as well as the talk-show right that assumes opposition to abortion means approving the war in Iraq.

There's also an increasing awareness among the Catholic hierarchy that America's political right has consciously used the abortion issue to cut the church off from most of the positions it traditionally holds in American society. That may be one reason that, in its last statement on the political obligations of Catholic voters, the Vatican warned: "The Christian faith is an integral unity, and thus it is incoherent to isolate some particular element to the detriment of the whole of Catholic doctrine. A political commitment to a single isolated aspect of the church's social doctrine does not exhaust one's responsibility toward the common good."

Whether Clinton or Obama comes out on top in the Democratic race, it's hard to see Catholic voters swinging toward John McCain, who is pro-war, pro-capital punishment, opposed to social spending and progressive taxation and in favor of stem cell research.

Tim Rutten is a Los Angeles Times columnist. Reach him at timothy.rutten@latimes.com.