Donald Trump has a big problem with Catholics. And he's earned it.

Hillary Clinton leads him by 19 points among weekly church-going Catholics and 16 points among other Catholics, according to the most recent Pew survey.

Not good for his prospects this Fall. Catholics are a quarter of the electorate and, four years ago, Mitt Romney split the Catholic vote almost 50/50 with Barack Obama -- and lost the election.

Why are so many Catholics down on Donald Trump?

One clear reason is that many are Latinos. Trump has made Latinos the villains of his campaign, so no surprise there.

But most American Catholics aren't Latino.

Most, like us, are descendants of immigrants from Ireland, Italy, Poland, Germany, or other European countries. We know our families' immigration stories -- which ancestors came to America and when, the stereotypes and discrimination they overcame, and the pride they took in their heritage and in America.

So, when Mr. Trump makes immigrants and immigration his enemy, he's attacking us and the families of most American Catholics. When he says build a wall to keep out Latin Americans and ban Muslims, he sounds just like the bosses who said "No Irish need apply" and the nativist politicians in the 1920s who passed laws to stop Italian immigration.

For good reason, John F. Kennedy, our only Catholic president, called the United States "a nation of immigrants." Mr. Trump has declared political war against that powerful and inclusive idea of America, so it's no wonder so many children and grandchildren of Catholic immigrants are not amused.

And then there is foreign policy.

For reasons that may not become clear unless he releases his tax returns, Mr. Trump is remarkably fond of Russian President Vladimir Putin. And remarkably weak when it comes to honoring our country's commitment to NATO -- that European alliance for mutual defense first led by General Dwight Eisenhower.

That concerns many Americans of all faiths. But it's particularly important to Americans with family ties to Poland, Hungary, Croatia, Ukraine, and other neighboring countries on the front lines of renewed Russian aggression. And, of course, these Americans are overwhelmingly Catholic.

Finally, there is Pope Francis.

Years before Mr. Trump declared war on Latinos, immigrants, and NATO, Jorge Mario Bergoglio, son of an Italian immigrant to Argentina, became Pope Francis I. He brought to the Papacy the New Testament spirit we learned in American Catholic schools -- tolerance, forgiveness, and love, particularly for refugees fleeing death or oppression. He spoke out on current issues from inequality to climate change -- and most American Catholics agree with him.

In recent decades, Catholics have become swing voters, generally supporting the winning presidential candidate but by small margins. 2016 could be different.

In large measure because of Mr. Trump, Hillary Clinton, raised in the social gospel of Methodism, could earn the largest share of the Catholic vote than any Democrat since JFK.

Is America a great country or what?

You don't have to be Catholic to understand Mr. Trump's danger to the American dream -- but it helps.

Martin O'Malley, former Governor of Maryland, is of Irish (no kidding) and German heritage. Jim Rosapepe, former US Ambassador to Romania, was born in Italy.