Jean Whitehead, 73, isn’t surprised that Southerners are now almost evenly split between voting for a Republican or a Democrat. To her, the South has always been more complicated than the conventional thinking would suggest.

A black retired school teacher in Birmingham, Alabama, she’s one of the people who proved the region isn’t a political monolith when she joined the majority in electing Doug Jones to the Senate last year — the first time a Democrat won a Senate seat in Alabama in more than 25 years.

Whitehead, who participated in the NBC News|SurveyMonkey Poll on attitudes of Southerners released on Thursday, said Republicans should be prepared to lose elections throughout the South in 2018, particularly because the region’s black residents have become more politically active since Donald Trump moved into the White House.

“I see more of an interest in voting,” she said. “I’m seeing more of an interest in young black people voting especially.”

The South has long been the most solidly conservative region in the nation, and its voters are a principal reason why Trump won the presidency. Of the 11 Southern states surveyed in the new NBC News poll, 10 voted for Trump in 2016, many by double-digit margins.

But the poll suggests that the Southern voters who showed up in 2016 are not necessarily representative of the region’s rapidly changing views. A majority of respondents said they supported same-sex marriage, legal status for undocumented immigrants and higher taxes for education and infrastructure. And many are cooling on Trump.

Pamela Wilson Cousins, a candidate for Jefferson County district judge, plants a campaign sign in Birmingham, Alabama, last month. African-American women are running in higher numbers in Jefferson County. Andrea Morales / for NBC News

In part, that’s because the population of the South has been shifting to more closely resemble that of the nation’s.

The 2010 Census concluded that the South is the fastest-growing region in the United States, adding 14.3 million people over a decade, for a total of 114.6 million, and Census estimates show that pace hasn’t abated. The South is home to 10 of the 15 fastest-growing cities in the country, which have seen population increases at an average rate of 9.4 percent since 2010.

That population growth has also brought demographic shifts. According to the Census, more than a quarter of the South's population is under 20. From 2010 to 2016, the black population in the region grew by 2 million, while the Hispanic population grew by 3 million, largely in Texas and Florida, according to William Frey, a demographer and author of “Diversity Explosion: How New Racial Demographics Are Remaking America.”

Black and Latino voters and young people in the South are friendlier to Democratic candidates than older, white voters are, Frey said, which could affect the region on Election Day.

“If it’s close between Republicans and Democrats in the South, that says more millennials are turning out to vote and more minorities are turning out,” Frey said. “It also means the white vote is not going to be as strong.”

Mike Espy, a former U.S. agriculture secretary, is running for the U.S. Senate in Mississippi. Rogelio V. Solis / AP

That’s why Mike Espy, an African-American Democrat who led the U.S. Department of Agriculture in the Clinton administration, believes he could win the Senate seat that Thad Cochran, a Republican, vacated last month in Mississippi — a state where black people make up nearly 38 percent of the population.

“[The shift] reflects the mood of the country where the parties are sitting at the poles,” Espy said. “In Mississippi that’s changing. People are really giving more consideration to their own economic interests and desires to improve along the lines of education, obesity, housing and income.”

Those issues matter more to voters than the party, Espy said, adding, “These interests are shared across party lines.”

It’s that economic appeal and the fatigue of partisan rancor in Washington that’s proven effective for Democrats in special elections since Trump entered the White House.