(CNN) The revelation last Thursday that House Speaker Paul Ryan had dismissed the Rev. Patrick Conroy, the Jesuit priest who had been serving as House chaplain, stirred an instant controversy on Capitol Hil l.

But a more revealing guide to the future of Catholic politics in the President Donald Trump era may have come a day earlier, when the US Conference of Catholic Bishops fired off a toughly worded letter urging Congress to restore legal protections for young immigrants who had been brought to the country illegally by their parents.

The timing of the two events was coincidental: Conroy, in fact, had been fired by Ryan two weeks earlier but did not reveal the dismissal until an interview with The New York Times last Thursday. But the juxtaposition was nonetheless revealing. The convergence illuminated a key question: As Trump's hard-edged agenda, particularly on immigration, draws more overt opposition from the leadership of the US Catholic Church, can the GOP maintain the dominant support among white Catholics it has usually enjoyed since the 1980s?

A key Republican voting bloc

White Catholics have been a critical component of the Republican electoral coalition since Ronald Reagan's era. Trump carried just over three-fifths of them nationally and, even more important, notably widened the Republican margin with them in several of the Rust Belt states that keyed his victory. But, already, polls show Trump losing more support, relative to his 2016 vote, among white Catholics than among white evangelical Protestants, another pillar of the GOP electoral coalition.

Conroy's collision with Ryan, himself a Catholic, may have reflected one aspect of the conflict between the Trump-era GOP and the church's leadership: questions of social equity in the party's approach to federal taxes and spending. According to Conroy, first Ryan's staff and then the Wisconsin Republican himself complained after the priest had urged, in a prayer on the House floor, "that there are not winners and losers under new tax laws, but benefits balanced and shared by all Americans." Ryan and his staff have denied that any individual prayer contributed to the dismissal.

New disagreements and a strong rebuke

But while Catholic leaders have frequently criticized Republican budget priorities as too harsh on the poor, spending is not the central arena of the building conflict between the church hierarchy and the Trump-led GOP.

Rather, immigration is the core issue heightening tensions. Underlying forces reshaping the church and the GOP alike are widening the distance between them. From one direction, Trump is pulling the GOP further toward his insular and racially infused nationalism, which is grounded in resistance to immigration in almost all forms. From the other, the Catholic Church's demographic transformation, with Hispanics representing a growing portion of its membership, is reinforcing its long-standing commitment to supporting immigrants and refugees.

"Based on the church teaching and history, the bishops have always been committed to immigration," says Kevin Appleby, the former longtime director of migration policy for the Catholic Bishops conference. "But ... in this particular era, where Trump has taken an anti immigrant position, they feel that their flock is being threatened and therefore their responses are more frequent and more exact against some of his policies."

In recent weeks, church leaders have advocated for immigrants and criticized Trump's agenda in a procession of bitingly critical documents.

In the letter to Congress last week urging legal protection for the so-called Dreamers, the Bishops' Committee on Migration wrote that the young people brought to the US illegally by their parents as children "truly exemplify the extraordinary contributions that immigrants can provide to our nation when they are permitted to reach their God-given potential."

In late March, the Bishops delivered an extraordinarily stern rebuke of Trump when they filed a legal brief in the Supreme Court case regarding the administration's executive order restricting immigration from several majority-Muslim nations. The executive order, the bishops charged, "has both the purpose and the effect of discriminating against Muslims." Then the brief added: "Such blatant religious discrimination is repugnant to the Catholic faith, core American values, and the United States Constitution. ... Having once borne the brunt of severe discriminatory treatment, particularly in the immigration context, the Catholic Church will not sit silent while others suffer on account of their religion."

In other recent statements, the bishops have criticized Trump for terminating the program that allows US parents to request reunification with their children from Central America, and have denounced his decision to send National Guard troops to the US-Mexico border. Pope Francis has also criticized Trump over the "Dreamers."

US Catholics are less white, more Hispanic

Such strong stands reflect long-standing church teachings, notes Appleby, who's now director of policy at the Center for Migration Studies of New York, a Catholic-affiliated think tank. But the bishops' militancy also reflects the church's new demographic reality, as Hispanics become increasingly central to its future. According to the Public Religion Research Institute, a nonpartisan think tank, in 1991 fully 87% of US Catholics were white. Today just 55% of US Catholics are white, while 36% are Hispanic. The direction of the change appears irreversible. While whites compose over three-fourths of Catholics 65 or older, the institute found, they account for only a little more than one-third of Catholics younger than 30. Hispanics (at 52%) now make up an absolute majority of such young Catholics.

Though the church has been relatively slow to move Hispanics into its top American leadership, the magnitude of that demographic change seems certain to solidify its commitment to defending immigrants and refugees in the years ahead.

"The church has an institutional interest in immigration, and it's something the church wouldn't apologize for," Appleby says. "If you have your flock, you have a pastoral care duty for your flock, and part of that duty is to advocate on their behalf and defend their rights."

Even as the Catholic leadership grows more committed to the immigrant cause, Trump is hurtling the GOP in the opposite direction, with an array of proposals to crack down on undocumented immigrants and reduce legal immigration. The key political question this raises is what this widening gulf means for the future political allegiance of American Catholics.

Hispanic Catholics tilt staunchly Democratic -- and traditionally much more so than Hispanic Protestants, many of which are evangelicals. In 2016, for instance, exit polls found Hillary Clinton beating Trump by 42 percentage points among Hispanic Catholics, compared with 30 points among Hispanic Protestants, according to figures provided by CNN polling director Jennifer Agiesta. In 2012, the gap was even more pronounced: Exit polls showed President Barack Obama carrying Hispanic Catholics over Mitt Romney by 54 points and Hispanic Protestants by just 14 points.

While Hispanic Catholics are a difficult target for Republicans, white Catholics remain central to the party's future. Like all white Christian groups, white Catholics are shrinking as a share of American society. According to Public Religion Research Institute data they have fallen from 16% of the total population in 2006 to 11% in 2016. (White evangelical and mainline Protestants have each declined by comparable amounts, the research shows.)

But white Catholics remain more concentrated in the Rust Belt states that are at the fulcrum of national politics: They compose about one-sixth of the population in Michigan, just under 1-in-5 in Ohio and Pennsylvania, and slightly more than one-fifth in Wisconsin, the Public Religion Research Institute found. In Iowa, they represent about one-eighth of the population.

Slipping support for Trump

Once viewed as the quintessential swing vote, white Catholics since the 1970s have usually leaned Republican. Jimmy Carter in 1976 was the last Democratic presidential nominee to carry a majority of them, though Bill Clinton squeezed out plurality victories among them when third-party candidate Ross Perot splintered the vote in Clinton's 1992 and 1996 wins. White Catholics broke for George W. Bush in both his 2000 and 2004 victories, and though Barack Obama narrowed the Democratic deficit among them to just 5 percentage points in 2008, they gave Romney a crushing victory of 59% to 40% over him in 2012.

Trump improved slightly on that in 2016, beating Hillary Clinton nationally among white Catholics by 61% to 37%. More important for Trump is that, according to figures provided by Agiesta, he significantly expanded the Republican margin compared with 2012 among white Catholics in Ohio, Michigan and Iowa. He beat Clinton by at least 20 percentage points with them in each state. (Exit polls didn't offer 2016 results by religion in Pennsylvania and Wisconsin.)

But recent Public Religion Research Institute polling found that white Catholics nationally now divide exactly 50-50 on whether they have favorable or unfavorable views toward Trump. That slippage presents a stark contrast to his robust standing among evangelical Protestants, fully three-fourths of whom viewed him favorably in the latest survey by the institute; that was his best showing among the group since taking office. Among white Catholics, Trump faces a cavernous gender gap: While three-fifths of the men view him favorably, a mirror image near three-fifths of the women are unfavorable to him.

Catholics vs. evangelicals

Daniel Cox, the institute's research director, says white Catholics are tugged between two impulses in their reaction to Trump. On the one hand, Cox says, "to the extent Trump frames his policies on protecting Christian, and particularly white Christian, status" he finds an audience among many white Catholics, if perhaps not as wide an audience as among white evangelicals.

But, Cox adds, compared with evangelicals, "white Catholics express greater comfort with cultural and religious pluralism. This is likely due to their own history as immigrants and outsiders trying to gain acceptance in the US."

The institute's polling repeatedly underscores the point. While three-fifths of white evangelicals supported Trump's temporary ban on immigration from Muslim-majority nations, white Catholics split exactly in half on the idea; likewise, while three-fifths of white evangelicals supported Trump's border wall, a 55% majority of white Catholics opposed it. In each case, white Catholic women were much more skeptical than the men. More broadly, Cox notes, while 53% of white evangelicals in a 2015 Public Religion Research Institute survey said "being a Christian" is a very important part of being American, only half as many white Catholics agreed.

Conservative views on social issues, led by abortion, continue to bind many white Catholics to the GOP, and Cox believes the even split over Trump evident among them in the latest polling by the institute is about the floor for the President with them. But even a decline for the GOP to that level could leave them short in closely balanced Rust Belt states where white Catholics remain most concentrated. Republicans have braced themselves for a backlash among Hispanics for Trump's restrictive impulses on immigration. But if the intense opposition to Trump's views about immigrants among Catholic leaders triggers any meaningful erosion among the rank-and-file in the pews, that could prove an even more costly price for his turn toward nativism.