A new paper from the National Bureau of Economic Research has confirmed an important but underappreciated fact of American history: in the 20th century, immigration shifted our politics permanently to the left. “European immigrants brought with them their preferences for the welfare state,” authors Paola Giuliano and Marco Tabellini argue. “This had a long-lasting effect on the ideology of U.S. born individuals.”

Long-lasting is right. The paper found that counties with high levels of immigration between 1900 and 1930 were more likely to be liberal today, nine decades later. Even controlling for factors like industrialization and urbanization, residents of these counties were more likely to favor redistribution, oppose spending cuts, and identify with the Democratic party.

This is old news to any student of history. The Democrats have been the immigrant party since before the Irish potato famine. The real puzzle is why. Why do immigrants so consistently favor the left, and why does that effect persist decades later, after they and their children have Americanized in every other respect from language to education to employment?

The sheer longevity of the effect refutes the most obvious theory, that immigrants are poor and poor people favor redistribution. In 1952, most American Catholics were immigrant-descended but far removed from any firsthand memory of tenement life, and Democrats still outnumbered Republicans among Catholics three to one. Today, South Asian immigrants are wealthier and more educated than natives on average, and they favor Democrats as lopsidedly as Hispanic immigrants do.

Giuliano and Tabellini hypothesize that immigrants import political preferences from their home countries. That theory has a long pedigree going back to Thomas Jefferson, who worried that immigrants from European monarchies “will bring with them the principles of the governments they leave, imbibed in their early youth… These principles, with their language, they will transmit to their children.”

But it doesn’t capture the whole picture. Giuliano and Tabellini cite the New Deal as the banner example of their thesis that more immigration means more redistribution, for obvious reasons. It was a massive episode of redistribution, and it happened when immigrant voting power was at its peak just after the Immigration Act of 1924. Their instincts are more right than they know. The closer we look at the New Deal, the more we see just how deeply entangled immigration is with the growth of the welfare state, in ways that go beyond demographics.

The groundwork for the New Deal was laid by two different political movements, both of which owed their existence to immigration: patronage and progressivism. The former was mainly an immigrant invention. Before 1850, patronage in America mostly meant replacing Whig postal clerks with Jackson men and vice versa. It was the Irish bosses of the big city machines that raised patronage from a tool into a system. Tammany Hall pioneered (in America at least) the mass distribution of favors and largesse as a basis for long-term political power.

The Progressives, by contrast, were generally not immigrants. Quite the opposite—and that is the point. Progressivism was a reaction to immigration by old stock Americans. The appalling conditions in slums and sweatshops led the progressives to favor a more active government, and they were more willing to tolerate state intrusion into new facets of life because their proposed beneficiaries were immigrants who, in their eyes, were powerless to help themselves. Jane Addams felt comfortable micromanaging the most intimate habits of Hull House residents because the ethnic difference between them made it easier for her to be patronizing.

Franklin Roosevelt had ample opportunity to observe both these traditions as governor of New York, and when he got to the White House he drew on both to staff his administration. It was not just that immigrant voters were a crucial part of the New Deal electoral coalition, as the NBER paper says. It was that the New Deal was invented and implemented by people whose politics were shaped by immigration, from Tammany veterans like Jim Farley to longtime progressives like Frances Perkins.

What lessons for today can be drawn from Giuliano and Tabellini’s paper? The first is that immigrant attachment to the left has very little to do with anything the opposition party does. Immigrants and their descendants consistently favor Democrats, regardless of whether the GOP candidate has courted them or spent his campaign railing against popery. Both Goldwater and Nixon believed that Catholic voters were natural conservatives, and both were snubbed by Catholics on election day despite their best efforts at outreach.

The second lesson is that immigration’s effect on domestic politics is not talked about enough. Extensive research has been done into how immigrants affect American labor markets, wages, culture, and cuisine, but “to the best of our knowledge,” Giuliano and Tabellini write, “we are the first to systematically document a similar impact on economic preferences and on political ideology.”

It’s no wonder that the topic is under-researched, since it raises doubts about the partisan political advantage that Democrats derive from immigration. The NBER paper suggests that immigrant bias in favor of Democrats has historically been due to factors that preceded their arrival on American shores. If that’s the case, then the problem isn’t anything Republicans have done or failed to do. The liberal advantage is simply built in.

California today is what New York State was a century ago: a portent of the future. Many assume that, as immigrants and their children assimilate into American culture in language and other externals, their politics, too, will come to resemble the American average. Giuliano and Tabellini’s paper suggests that this is wrong. They found that Ellis Island era immigration was still boosting Democrats a century later. If California’s Democratic advantage is similarly durable, it will remain a one-party state for a long time no matter what Republicans do, and so will any other state that follows California’s demographic path.

At a campaign rally in Madison Square Garden in 1936, Franklin Roosevelt told the crowd that opponents of the New Deal were “already aliens to the spirit of American democracy.” Doubters should get out, he thundered. “Let them emigrate and try their lot under some foreign flag in which they have more confidence.” Ironically, the same factors that brought Roosevelt victory have ended up making his rhetorical flourish come true. After a century of immigration pushing America’s political center of gravity to the left, it is big-government advocates who dominate the political spectrum and skeptics of the welfare state who are aliens in their own country.