Immigration, a wit has said, is the sincerest form of flattery. This dispirited nation needs some of that, so President Biden has wisely made immigration reform his initial legislative proposal.

The nation also needs a healthy opposition party, and the impending immigration debate will give the Republican Party an early opportunity to rehabilitate its reputation by adopting policies unlike those of Biden’s predecessor, who propelled his ascent to the presidency by stoking anxieties about immigration. Congressional Republicans will have to choose between aligning with the animosity of constituents who misunderstand how this nation has prospered by assimilating 100 million immigrants, or with the generosity of the United States’ majority.

David J. Bier and Alex Nowrasteh of the Cato Institute report that, for the first time in Gallup’s 55 years of polling on the subject, “more Americans support increasing immigration than decreasing it.” Support for decreasing it has plummeted from 50 percent in 2009 to 28 percent today. Last year, 77 percent called immigration “a good thing,” and a similar majority today favor a path to citizenship for “dreamers,” those who were under 16 when brought here before 2007 by parents who were not lawful residents.

About 40 percent of unauthorized immigrants came not through porous borders but on visas they overstayed. Of the approximately 11 million (down from 12.3 million in 2007), 62 percent have lived here at least 10 years, 21 percent at least 20 years. Of the more than 5 million children under age 18 living with at least one unauthorized immigrant parent, more than 4 million, having been born here, are citizens. The 11 million are not going home. They are home. And Americans’ decency would prevent the police measures necessary to extract them from their communities.

Biden’s predecessor said “our country is full,” although there are 145 countries and territories with greater population densities. Two-thirds of Americans live in cities that occupy 3.5 percent of the land. In 80 percent of America’s counties, the number of prime-age workers (25-54) declined between 2007 and 2017. As a candidate, Biden proposed “a new visa category to allow cities and counties to petition for higher levels of immigrants” for economic reasons.

Bier and Nowrasteh report that America’s per capita immigration rate today is “as close to zero as it has ever been.” The nation now has a declining birth rate and an aging population that is retiring, at a rate of 10,000 a day, into Social Security and Medicare systems that are unsustainable without a workforce replenished by immigrants. Furthermore, a steady influx of them will enable the U.S. economy to regain, late in this century, its place as the world’s largest economy as China’s workforce shrinks, a debilitating echo of the 1980-2016 one-child policy.

The debate about immigration that Biden is reigniting, and especially his proposed path to citizenship for the 11 million, implicates the nation’s understanding of itself. And it will roil a dark current of 21st-century politics, concerning which some 19th-century history is germane.

The years 1845-1855 produced the largest single-decade increase in the foreign-born percentage of the U.S. population. Three million immigrants arrived in a nation whose population was 23 million — the equivalent of 42 million arriving between 2000 and 2010, when 14 million actually did. In 1858, when Abraham Lincoln said that half the Americans then living were born elsewhere, immigrants were one-thirdof the approximately 9,400 residents of Springfield, Ill.

Seven years later, Lincoln was buried there after a nation-saving Civil War victory that had been substantially aided by immigrant soldiers. “There are those damned green flags again,” said Confederate Gen. George Pickett as he watched an Irish unit prepare to attack. Ireland’s potato famine helped to doom the Confederacy. Recruiting posters were printed in foreign languages, and the 1862 Homestead Act was publicized around the world to attract immigrants, 800,000 of whom came during the war. Historian Jay Sexton in “A Nation Forged by Crisis” says about 25 percent of Union soldiers and 40 percent of seamen were foreign-born. Union officials cast the war as an episode in a larger struggle for republican government, here and elsewhere, thereby, Sexton says, “decoupling the idea of the nation from Anglo-Saxon Protestantism.”

Today, anti-immigration sentiment is disproportionately concentrated among recent Republican voters who are timid nationalists dismayed by the decoupling of the nation from their conceptions of it. Strangely, they fear that the United States cannot be itself if it is as welcoming to immigrants as it was when they were making the United States the success that it is.