On a Tuesday evening in the nation’s capital, a clock chimes loudly just as Harold Holzer is about to make a point.

“There’s Lincoln, echoing,” Holzer says. The quip draws laughs from a crowd packed into the parlor of a historic home to hear what Holzer has to say about the 16th president. It’s a joke, but there’s truth to it, too.

It’s been nearly 160 years since the last summer Abraham Lincoln spent living in this 34-room Gothic Revival home. Now, the building houses a nonprofit known as President Lincoln’s Cottage. And it’s become a “museum of ideas” dedicated to continuing conversations that began here during his presidency.

For Holzer, it’s a fitting spot to share insights from his latest book about Lincoln, which argues that, for generations, scholars have overlooked an important part of the popular president’s legacy.

In addition to his well known role leading the Union through the Civil War, Lincoln was a “history-making leader” on another front as well, according to Holzer. And in “Brought Forth on this Continent: Abraham Lincoln and American Immigration,” the historian makes his case.

Even Holzer, who’s written and edited more than 50 books — many of them about Lincoln — says he was surprised by what he learned as he researched the issue more deeply.

“He deserves enormous credit for staving off the forces of fear and bigotry and envisioning a government of, by, and for the people, regardless of their national origin,” Holzer writes in the book’s introduction.

At the Washington event, Holzer tells the audience that in public remarks before and during his presidency, Lincoln consistently recognized how vital immigrants were to the country’s present and future.

In 1858, for example, the Republican Senate candidate gave a speech to a largely German crowd describing immigrants as “the blood of the blood” of America’s founders.

“At a key moment, he doesn’t say immigrants were ‘poisoning the blood,’” Holzer says, referencing recent comments espoused by former President Trump on the campaign trail. “They’re enriching the blood.”

CNN spoke with Holzer about Lincoln’s approach to immigration, how his views evolved, and how the author’s own family history shaped his work. The interview has been edited for length and clarity.

Why do you think it’s important to talk about Lincoln’s immigration legacy?

I thought it was important — especially since it’s an unsolved problem, a seemingly insoluble problem in America — to go back and see how political leaders reacted to the challenge of diversity and immigration in the 19th century. And sure enough, Lincoln is there on the issue.

Not always liberal, privately, as he worries about Irishmen who come here and vote heavily Democratic leaning against him and his interests of anti-slavery. But he’s always embracing the concept, in public, of open immigration. And he loves German immigrants because they vote Republican. And they’re going to be an essential part of his emerging coalition to fight slavery.

Unfortunately, he feels he also has to woo anti-immigration nativists in that coalition. Once the Know-Nothing Party begins to dissipate after 1856-57, he wants them with him, so he doesn’t denounce nativism openly. He kind of woos their disaffected members into his coalition. He never says the wrong thing, but there’s an undertone of turning the other cheek.

You talk about how Lincoln’s thinking really evolved on immigration. So if this tacit acceptance of nativism is where he started, where does he end up and how does he get there?

He consistently believed in open borders and open access, and the principle that if you get here and you’re here for five years, you can become a citizen, and that in the interim you should be encouraged to work.

The evolution comes as kind of a practical reaction to the realities of the rebellion. He is suddenly faced with the prospect of a civil war. And he did not want it to be an exclusively Republican Party war. So he immediately recruited Democrats to lead regiments.

In that same vein, he recruited Irish and German and other ethnic group regiments, because he wanted the might of the immigrant community to be brought to bear against the rebellion. You know, 23% of the federal army spoke with a foreign accent.

Then, ultimately, Lincoln comes to terms with the sad fact that the war has taken hundreds of thousands of lives. And also cost maybe hundreds of thousands more in their ability to work. So, publicly, in speaking to Congress in the equivalent of a State of the Union message, he asked for federal involvement in immigration — not to prevent it, but to encourage it. He called it an Act to Encourage Immigration. And he’s pretty up-front about the reason why. Not only did they need more manpower, but he said they had a reduction in workers in agriculture, in factories, and in the mines.

So he wanted new Americans. And he was prepared not only to encourage them, but at one point he proposed paying for their ocean passages with federal money. That was a bridge too far for Congress. But he did propose it. It was an extraordinarily radical position at the time.

I feel like the average American today does not realize what a big role immigrants played in fighting the Civil War.

I agree; 90 Medal of Honor winners among Germans, more than 40 among Irish. These are new citizens or not-yet-citizens; 200,000 German recruits in the army, 150,000 Irish. They were vital. The Germans fought at Gettysburg and Chancellorsville for the Union. The Irish were heroic in both defeats and victories, particularly at Gettysburg, where the “Fighting Irish” kind of got their reputation.

And if you believe Robert E. Lee’s kind of self-serving observation that the South lost the Civil War because they were overcome by just the sheer manpower advantage of the North, well, this is the manpower. It’s the immigrant community that provides the difference.

You write that Lincoln “not only won the Civil War fight over slavery, but he beat back the even longer rebellion against immigration.” What do you mean by that?

I think Lincoln won the Civil War, but like all civil wars that are civil rights wars as well as the wars for racial equality and open access to the United States, they were temporary victories or half-way victories, because the battles have gone on ever since.

Where immigration was concerned, the law that Lincoln fathered in 1865, although he didn’t get to sign it because he died, was the last pro-immigration law passed in the United States for 100 years until Lyndon Johnson signed (the Immigration and Nationality Act eliminating) quotas in 1965.

I saw that you dedicated the book to your grandparents, who were all immigrants from Europe. Is your own family’s immigration story one reason you wanted to explore this topic?

I wanted to pursue this not only because it’s the neglected chapter of the Lincoln story, but because I kind of felt I owed a debt to my own ancestors, to acknowledge their courage and persistence — and also America’s open door that allowed them to arrive and thrive here.

When we talk about immigration, what lessons does Lincoln’s legacy hold for today’s leaders?

I don’t love what the professionals call presentism, thinking that there are exact parallels. Our country is not much bigger geographically than it was in the 1850s, and it’s ten times more crowded with people.

But I still think the takeaway is that America is the better for attracting strivers who want to do work, who want to start at the bottom and rise as far as their talent and ambition will take them, and that we’re not being soaked by immigrants, we’re being treated to the fact that they want to give. And I think we’ve just lost sight of that.

And maybe Lincoln, who illuminates the body politic on so many levels, and articulates things so beautifully, can remind us about what our open country produced. They produced the Biden family, out of Ireland, and the Trump family or the Drumpf family out of Germany, people who came here to do better here. And, you know, that’s our American story. And I wish we wouldn’t close our eyes and our hearts to it.

You’ve written so much about Lincoln. When you researched this book, was there anything that surprised you or made you see something in a new light?

I was astonished at how far he was willing to go to encourage immigration. I really hadn’t thought about it because I was like everybody else who deals with Lincoln. I’m immersed in, you know, how he wants to unite the “house divided.” Well, it turns out he didn’t just want to unite the house divided. He wanted to expand and diversify the “house divided.”