At Tuesday’s Democratic debate sponsored by CNN and the Des Moines Register, nobody seemed to notice the elephant in the room—or perhaps the candidates and moderators just didn’t want to acknowledge its presence. Whether it was out of blindness or stubbornness, it tells us a great deal about the state of the Democratic Party in our time—and also about the state of American politics.

That elephant is immigration, and the issue it represents is the defining one of our time. It is the most intractable, the most emotional, and the most irrepressible of all matters facing Western societies. And yet it was almost totally ignored in the most crucial debate so far in the Democratic quest for a presidential nominee. Two passing references was all the issue got over two hours of polemical fireworks.

President Trump certainly came in for his share of opprobrium from the top six Democratic candidates, yet nobody seemed to have the slightest awareness that the single most important issue driving Trump’s political rise four years ago was immigration. A Pew Research Center survey revealed after the 2016 election that 66 percent of Trump supporters considered immigration to be a “very big” problem, the highest percentage for any issue. For Hillary Clinton supporters, the corresponding percentage was just 17. Also, fully 79 percent of Trump voters favored building the border wall he advocated, compared to just 10 percent for Clinton supporters.

During the 2016 campaign, the Washington Examiner called immigration “the mother of all issues”—touching on jobs, national security and terrorism, the public fisc, and the cultural definition of America. That latter factor, said the paper, was a “nearly existential question” involving the ultimate definition of a nation without borders.

Elsewhere in the West, we see the same political percolation. By most analyses, immigration was the driving force behind Britain’s 2016 vote for Brexit. The Atlantic ran a piece in June of that year headlined: “The Immigration Battle at the Heart of Brexit.” After the vote, Slate rushed out to interview former British prime minister Tony Blair—who, as the website noted, “presided over the opening of Britain’s borders.” That had unleashed “a wave of immigration unprecedented in [Britain’s] history.” Within a few years, noted Slate, “roughly twice as many immigrants arrived in the United Kingdom as had arrived in the previous half-century.” The Brexit vote was in large measure a rebuke to that Blair project, pushed avidly and relentlessly by the British ruling class.

Elsewhere in Europe—Hungary, Poland, France, Germany, Italy, even Sweden, among other nations—mass immigration has emerged as the dominant issue, roiling the waters of national politics and pushing to the fore various types of conservative populism. New parties have emerged to join the issue, and old parties have gained new sway.

Many commentators and political analysts in recent years have posited the idea that a new political fault line has emerged throughout the West, between the globalist elites and ordinary citizens who are more nationalist in their political sensibilities and more culturally protective. This is true. And while there are many issues that have come into play here, such as trade, military adventurism, identity politics, and political correctness, immigration is the key driver.

Generally, the open-border elites have been on the defensive since Donald Trump seized the issue in 2015 and tied it to the emotional matters of terrorism and crime. Trump was probably correct in the first Republican debate of the 2016 election cycle when he said that, were it not for him, immigration probably wouldn’t have been a major topic of discussion. It certainly seemed as if the other candidates preferred to keep it out of the campaign debate so it could be handled after the election in the more controlled environments of Congress and the courts. By bringing it up, even in his crude and disturbing manner, Trump galvanized a large body of voters who had concluded that the elites of both parties didn’t really care about controlling the borders.

Indeed, in their 2018 book, The Great Revolt, Salena Zito and Brad Todd posit that Trump got an extra boost from working class Americans put off by the attacks on him from prominent politicians of both parties who called his immigration concerns “unhinged,” “reprehensible,” “xenophobic,” “racist,” and “fascist.” Zito and Todd write that many Trump voters “saw one candidate, who shared their anxiety about immigration’s potential connections to domestic terrorism, being attacked by an entire political and media establishment that blew off that concern as bigotry.”

In this great political divide, the Democratic candidates at the debate represent the elite preference for policies that embrace or nearly embrace open borders. An NPR study of candidate positions indicated that, on the question of whether illegal crossings should be decriminalized, four of those on the debate stage say yes, while the positions of the other two remain “unclear.” On whether immigration numbers should be increased, four say yes, while two are unclear. On whether federal funding for border enforcement should be increased or decreased, five have no clear position, while one says it should be decreased. A separate Washington Post study on the candidates’ views as to whether illegal immigrants should be covered under a government-run health plan found that five say yes while one has no clear position.

The Democratic Party has become the party of the country’s elites—globalist, internationalist, anti-nationalist, free-trade, and open borders. Those views are so thoroughly at variance with those of Trump voters that it is difficult to avoid the conclusion that we have here a powerful issue of our time, perhaps the most powerful issue. Yet the journalistic moderators at Tuesday’s event didn’t see fit to ask about it. And the candidates weren’t inclined to bring it up in any serious way.

Perhaps they thought that if they just ignored that elephant, eventually it would go away. It won’t.

Robert W. Merry, longtime Washington, D.C., journalist and publishing executive, is the author most recently of President McKinley: Architect of the American Century (Simon & Schuster).