The United States' first black president spent a fair bit of time talking about the other side of his family tree during his speech Wednesday night at the Democratic National Convention, recalling his Scots-Irish grandparents and their predecessors in some detail:

See, my grandparents -- they came from the heartland. Their ancestors began settling there about 200 years ago. I don't know if they had their birth certificates, but they were there.
They were Scotch-Irish mostly -- farmers, teachers, ranch hands, pharmacists, oil rig workers. Hearty, small-town folks. Some were Democrats, but a lot of them, maybe even most of them, were Republicans -- the party of Lincoln. And my grandparents explained that folks in these parts, they didn't like showoffs, they didn't admire braggarts or bullies.

The inclusion of their heritage might have seemed like an aside -- a bit of rhetorical paint for the picture Obama was drawing -- but in the Year of Trump, it's no coincidence. Obama was tapping into a reemerging cultural identity that goes a long way in explaining the rise of Donald Trump and the current polarization of American politics.

As we wrote last year, the Scots-Irish who have dominated the American South and Appalachia -- the first white settlers in the American heartland -- were actually mostly Yellow Dog Democrats. They were populists who were persuaded to vote for Ronald Reagan but then went back to Bill Clinton in the 1990s.

Today, they are almost as united in being Republicans as they were for Democrats. And Trump's sweeping of basically the entire South and Rust Belt in the GOP primaries and his unusual amount of success with working-class white voters in general-election polls speaks to the continued Scots-Irish influence on American politics. This is a group that has continued to swing to the right and has found a champion in the brash, billionaire outsider known for his populism and red-meat politics.

But without mentioning Trump directly, Obama's speech recalled a day when this demographic didn't have much regard for the likes of Trump. And a question hanging over the current battle for the White House is whether Hillary Clinton has any hope of winning back the support her husband secured from the Scots-Irish.

In a 2004 Wall Street Journal column, Jim Webb, the former senator from Virginia and 2016 Democratic presidential candidate, warned of the danger of overlooking that key group of voters -- the "borderers" who trace their history back across the Atlantic to Ulster and before that to the scrappy Calvinist borderlands of England and Scotland.

"Few key Democrats seem even to know that the Scots-Irish exist, as this culture is so adamantly individualistic that it will never overtly form into one of the many interest groups that dominate Democratic Party politics,” Webb wrote.

And although Census data suggests that just 2 percent of the population self-identifies as Scots-Irish, that low figure fails to reflect the strong historical and cultural ties among a distinct group of people, many of whom refer to themselves as simply "American."

They "don’t go for group-identity politics any more than they like to join a union,” Webb wrote in his 2004  book "Born Fighting: How the Scots-Irish Shaped America."

Politicians ignore the Scots-Irish at their peril. According to Colin Woodard, author of "American Nations: A History of the Eleven Rival Cultures of North America," the Scots-Irish and the regions that they influenced are an enormous part of the country: "The region that they left the dominant cultural footprint on is the largest of the regional cultures."

Indeed, according to Woodard, the "Greater Appalachia" region is the biggest of the 11 American nations, at more than 56 million people. It stretches from Pennsylvania to New Mexico.

That's because the Scots-Irish didn't stay put in Pennsylvania, where many of them arrived in the 18th century. They migrated down the Shenandoah Valley, building log cabins in the valleys and then following creeks up into the Appalachian Mountains, embracing a libertarian sense of individual freedom. They moved south and west, into parts of Ohio, Indiana, downstate Illinois and much of Missouri, as well as to the uplands of the South and on into the Ozarks, writing as they went the story of the American frontier. They shaped an enduring American identity of individualism, helping to elect presidents with whom they shared that heritage: Andrew Jackson, James Polk and Woodrow Wilson.

The region's libertarian ethics and distrust of government have led it to be overwhelmingly Republican in recent decades.

"This was Obama's worst region by far," Woodard points out.

But it is also a region that prides itself on the values of hard work and common decency that the president played up Wednesday in his convention speech.

"They knew these values weren’t reserved for one race; they could be passed down to a half-Kenyan grandson, or a half-Asian granddaughter," Obama said. "In fact, they were the same values Michelle's parents, the descendants of slaves, taught their own kids living in a bungalow on the South Side of Chicago."

And Obama, Woodard contends, is correct to see the potential and need for Democrats to win back some support among the Scots-Irish in 2016. Trump is not particularly libertarian or oriented toward individual freedom, he said. In fact, Woodard said, neither Trump nor Clinton "is clearly a slam dunk for the greater Appalachian traditions."