Pundits have been trying for weeks to explain why Donald Trump has continued to lead Republican polls, drawing massive crowds and attracting a media circus, despite stepping in it time and again on camera. But the Donald tweeted something yesterday that may help answer all their questions.

While the tweet reads as nothing more than Trump’s typical bombast—more bluster than substance—the Republican frontrunner seems to be invoking a secretive political organization that dates back to the first days of the Republican Party: The Know Nothings.

The Know Nothing movement emerged during the years leading up to the Civil War as a few politicians searched for ways to build a new political party that could compete with the Jacksonian Democrats who had dominated American politics for decades. The group’s name purportedly comes from its somewhat secretive party structure. When asked about its activities, members were supposed to respond: “I know nothing.” 

The Know Nothings were rabidly xenophobic. Catholic immigrants from Ireland and Italy had flooded the United States during the 1840s and 1850s. This was the first time that Americans had to confront an immigrant class whose origin stories, last names, and religious beliefs set them apart from the British settlers who had cleared her forests and populated her first towns centuries earlier. The Know Nothings claimed that Catholics immigrants owed their allegiance, not to the burgeoning American government, but to the Pope, whose autocratic ruling style was antithetical to American democratic values (the same argument that was trotted out when John F. Kennedy was campaigning for president). And for a country whose founding fathers had written about religious tolerance only a few decades earlier, the message was surprisingly effective.

The movement took hold of the country, establishing particularly strong footholds in the Northeast. At its peak in 1854, the party (which had taken the name American Party in 1849) got 52 candidates elected to the House of Representatives under the Know Nothing banner in a single election year. That landslide propelled Nathaniel Banks—who would later become Speaker of the House, a prominent union general, and governor of Massachusetts—to Congress for the first time. That Milliard Fillmore, the 13th president of the United States who was ousted from the White House in 1852, ran for president four years later as a Know Nothing demonstrates their influence at the time.

Elected Know Nothings installed sweeping programs to trample any Catholic influence on American politics. They required reading the Protestant bible in schools, purged Catholic objects from public buildings (including the Washington Monument, where Know Nothings led a covert mission to remove and destroy a stone sent by the Pope in 1854), and moved to prevent Catholics from holding public office. In Massachusetts, where the party controlled both the state house and the gubernatorial mansion, the Know Nothings convened a “Nunnery Commission” designed to ferret out the dangers posed by nuns to society at large. According to John R. Mulkern’s book on the Know Nothings, the commission was disbanded after the Boston Daily published a scintillating expose of the exploits of the commission: Joseph Hiss, the “Grand Worshipful Instructor” of the Commission, had apparently made suggestive remarks to two nuns; his compatriots had laid in wait in the attic of a Catholic school near Boston, popping out to terrify the children.

As slavery eclipsed immigration as the political hot topic in the late fifties, the Know Nothing movement started to decline, and it was the (then still very young) Republican Party that eventually subsumed the movement, its nativist bent muffled, for the most part, by Lincoln, who spoke out sharply against it.

Since the Civil War, Know Nothings have appeared in American discourse, persisting in popular culture or in charges leveled at fringe members of the Republican Party. In Gangs of New York, Daniel Day Lewis played William Poole, a real Know Nothing leader in Manhattan during the 1850s. When Republican House Representative Eric Cantor lost his reelection campaign in Virginia, Paul Rosenberg wrote that it represented a triumph for the wing of the Republican Party that still adheres to Know Nothing nativism. Timothy Egan has written that the birther movement, which claims that President Obama was born outside the United States, was “building a nation of Know Nothings.” 

This brings us back to Donald Trump, himself a candidate who has catapulted to the top of the Republican field by stoking fears about an influx of immigrants. Just last week, conservative pundit Charles Krauthammer accused Trump of running his campaign on “know-nothing xenophobia” that would “damage” the Republican Party. 

Like his predecessors the Know Nothings, Trump faces a challenge: how to win supporters from an electorate that, for the last eight years, has seemed to gravitate more and more toward the Democratic Party. Like the Know Nothings, Trump has chosen to denigrate immigrants, playing up the xenophobic idea that they are stealing jobs and resources from American citizens. He even went so far as to accuse Mexican immigrants of raping American women.

The rise of the Know Nothings, an episode in American history often brushed under the rug or simply forgotten, demonstrates that Trump is a part of a tradition dating to the earliest days of the Republican Party. The fear of immigrants has long driven American politics, bringing together coalitions that have propelled even the most unlikely candidates to the halls of American political power. If nativist sentiment continues to rise, just as it did in 1854 when the Know Nothings swept Congress, Trump could be a candidate to be reckoned with. And if he runs as a third party candidate—a prospect that has Republican leaders quaking in their boots—maybe Trump will choose to run as a “Know NOTHING!”