By JACOB VIGDOR

Ordinary events aren’t usually newsworthy, and North Carolina’s adoption this week of a constitutional amendment banning gay marriage and civil unions was an ordinary event. Twenty-nine other states already have similar bans, and North Carolina’s referendum passed by a tally that was both overwhelming (61%-39%) and almost perfectly anticipated in polls. Yet the run-up to this event was front-page news nationwide.

The media frenzy created awkwardness for President Barack Obama, as Democratic Party elders and members of his own cabinet drew attention to his “evolving” views on gay marriage. Ultimately, Mr. Obama was compelled to speak out on the issue the day after the election, bringing his evolution to a close by offering some carefully measured sentences of support to ABC News.

Why was North Carolina, of all places, the state that made the president abandon his “don’t ask, don’t tell” stance on gay marriage?

The answer is that the Old North State has become a battleground. Mr. Obama won the state narrowly in 2008, and Democrats will descend upon Charlotte in September for their national convention. Republicans, meanwhile, have captured both houses of the state legislature for the first time since Reconstruction and hope to follow in November by taking the governorship and the state’s 15 electoral votes to Mitt Romney.

No one is taking the state for granted anymore—least of all the voters. Nearly 40,000 more voters cast ballots in this year’s primary than in 2008, though the presidential nominating contests for both parties are settled.

This is surprising given that for much of North Carolina’s history, elections have been afterthoughts. Between 1876 and 1964, Democrats dominated the state legislature, the governor’s mansion and national elections, breaking only to select Republican Herbert Hoover over Democrat Al Smith in 1928. After the Lyndon Johnson administration, the state—like much of the South—turned toward the GOP in national elections while retaining Democrats in most state offices. Between 1968 and 2004, Jimmy Carter (from neighboring Georgia) was the only Democratic presidential candidate to win the state.

Even the Andy Griffith Show attests to the irrelevance of elections in North Carolina: Ostensibly a chronicle of a North Carolina elected official, the show introduced a re-election plotline only once in 249 episodes.

North Carolina’s long-time political stasis reflected demographic stasis—and now its state of flux reflects demographic flux. In 1960, five of every six North Carolina residents had been born there, and only one in 200 had been born outside the United States.

Associated PressVoters in Brunswick County, N.C. , on Tuesday

Today, 47% of the state’s adult residents were born elsewhere, and more than 650,000 are immigrants—some highly educated and working in the state’s knowledge economy, but the majority being recent arrivals from Latin America. Tobacco farming, textiles and furniture manufacturing have receded as economic engines, displaced by financial services and high-tech industry. The state’s mill towns are shrinking as the major cities are expanding, including Charlotte and the so-called Research Triangle area encompassing Raleigh, Durham and Chapel Hill.

These demographic trends were evident in Tuesday’s election. The anti-gay-marriage amendment carried 93 of the state’s 100 counties, and Charlotte and the Research Triangle accounted for five of the seven exceptions. In Chapel Hill—the town that the late conservative Jesse Helms allegedly proposed encircling with a fence to substitute for a state-subsidized zoo—some precincts voted against the amendment by almost 10 to 1.

The “Jesse Helms memorial fence” might soon need to encircle communities beyond Charlotte and the Research Triangle. Voters in the counties surrounding Asheville and Boone, two university towns in the Blue Ridge mountains, narrowly opposed the amendment. And the amendment passed only narrowly in counties dominated by two other growing cities, Greensboro and Wilmington.

This helps explain why the Republican speaker of the House, Thom Tillis, predicted in March that although the constitutional amendment would pass this year, it would be repealed within 20 years thanks to “generational” shifts.

There are other factors at play, too. Black voters, for example—key components of the Democratic coalition four years ago—revealed their socially conservative streak in the amendment vote. Voters in Halifax County, a majority-black rural area in the state’s northeast, supported Mr. Obama by a 2-to-1 ratio in 2008, but this week they supported the gay marriage ban by a similar ratio.

Then there’s the state’s business community, which included some prominent foes of the ban. In a two-minute YouTube video prepared for an anti-amendment group, Catherine Bessant, an executive at Charlotte-based Bank of America, referred to the proposed ban as “a direct challenge to our ability to compete nationally for jobs and economic growth.”

Perhaps because of such sensitivities, the Republican nominee for governor, former Charlotte Mayor Pat McCrory, expressed only token support for the amendment—telling the Charlotte Observer, “I’m in favor of it, and that’s all I’m commenting on because I’m concentrating on other issues.” With a state unemployment rate near 10%, exceeding the national average for more than four years, it isn’t hard to imagine what those other issues might be.

Tuesday’s vote shows that the economic doldrums haven’t dulled North Carolinians’ attention to social issues. More than anything else, though, the gay marriage vote reveals that the state’s voters can be united or divided along numerous lines. Come November, North Carolina’s electoral votes will go to the candidate who can best exploit these fault lines to his advantage.

Mr. Vigdor is a professor of public policy and economics at Duke University.