Much has been made of Donald Trump's problems with a few voting groups — female voters, blacks and Hispanics, and young voters, in particular. And, to be sure, they are all problems.

But relatively speaking, his biggest problem actually appears to be with a different group: Catholics.

Yes, the man who once feuded with the pope (how soon we forget that actually happened) is cratering among Catholics.

Back in 2012, GOP nominee Mitt Romney lost the Catholic vote by just 2 points, 50 percent to 48 percent. And the GOP has actually won the Catholic vote as recently as 2004 and in 5 of the last 10 presidential elections.

But Trump trails among Catholics by a huge margin. A new poll from the Public Religion Research Institute released this week shows him down 23 points, 55-32.

A Washington Post-ABC News poll released earlier this month painted an even worse picture for Trump's Catholic support. He was down by 27 points, 61-34.

If you compare the difference between Romney's margin among Catholics in 2012 and Trump's margin among Catholics this year, the 25-point difference is tied for the biggest shift of any demographic group in the Post-ABC poll.

(The only group that matches that 25-point shift is white, college-educated women. Romney won them by 6 points; Trump trails by 19.)

Trump's deficits among non-whites and young voters, by contrast, are similar to where Romney and Republicans have been in recent years. The Post-ABC poll, in fact, showed Hillary Clinton failing to match Obama's margin among nonwhites — though not in a statistically meaningful way — while her margin among young voters ages 18-to-29 was three points better.

These are groups, in other words, that Republicans don't expect to do well with. And they still don't.

But Catholics have long been a swing vote in presidential elections, and right now they're swinging hard for Clinton.

It's also hard to overstate just how significant Trump's poor performance among Catholics is. That's because they comprise about one-quarter of voters in the United States (25 percent in 2012 exit polls) and are about as big a voting bloc as non-whites (28 percent) and independents (29 percent).

While we often look at how Trump is doing worse than Romney among Hispanics, we're really talking about the difference between Trump taking 45 percent of the vote and 46 percent — or maybe 49.5 percent or 50.5 percent. That's because Hispanics are only about 10 percent of the electorate, and the GOP's share of that vote is likely to be between 20 and 35 percent or so.

When talking about Catholics, though, Trump is basically adding 5 to 7 percentage points to Clinton's overall margin. If 25 percent of the electorate is Catholic, Clinton is currently taking 14 to 15 points worth of that chunk, while Trump is taking 8 or 8.5 points. And this is a group, again, that is usually close to tied.

The reasons for Trump's struggles among this group are open to interpretation. Perhaps Pope Francis' criticism of Trump and Trump's surprisingly confrontational response have turned off Catholics to Trump's candidacy.

"A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," the pope said in February when asked about Trump's border wall.

As the Religion News Services' John Gehring recently posited, it could also have something to do with Trump's immigration policies:

"Part of Catholics' DNA is an appreciation for how Irish and other immigrants toiled and thrived in the shadow of a suspicious, fiercely anti-Catholic culture dominated by white Anglo-Saxon Protestants.

"When Trump calls for a religious test for Muslims entering the country; questions the faith of Hillary Clinton, President Obama and Mitt Romney; and demonizes undocumented immigrants as 'rapists,' it's a reminder of the ugly nativism that Catholics once faced.

"While this contemporary strain of old xenophobia is particularly felt by Latinos who increasingly are the face of the Catholic Church in the United States, many white Catholics surely take pride in family stories of grandparents and great-grandparents who were strangers in a new land. Trump has dug himself a deep hole he is unlikely to climb out of with these voters."

But whatever the cause, Trump's struggles among Catholics remains one of the really undersold story lines of the 2016 election.