(CNN) One key measure of any Democratic wave in the midterm elections will be whether it crests high enough to overcome the formidable Republican defenses in the growing suburbs across the South. The answer will have implications that extend far beyond 2018.

While Democrats have notched significant gains since the 1990s among white-collar suburban voters in most parts of the country, they have until recently made very little progress at loosening the Republican hold on affluent and increasingly racially diverse suburbs around such Southern metro areas as Atlanta, Houston and Dallas.

But suburban unease with Donald Trump's turbulent presidency may finally provide Democrats an opening to establish a beachhead in such places -- a development that would rattle the electoral map. Although the recoil from Trump among white-collar suburbanites inside the South is not as great as outside of it, both public and private polls signal that enough suburban voters are pulling away from him to create much greater opportunity than usual for Democrats this fall in the governor's race in Georgia, Senate races in Tennessee and Texas, and several suburban House seats across the region.

"The South is not immune," says Fred Yang, a Democratic pollster working in both Georgia and Tennessee. "We start off lower in some of these (Southern suburbs), definitely. But we are also definitely making inroads first and foremost with college-educated white women, but also college-educated white men."

Many of the most vulnerable Republican House seats around the country are centered on white-collar suburbs. Democrats have strong opportunities in suburban seats from New York, New Jersey and Philadelphia through Northern Virginia, Chicago, Minneapolis, Detroit, Denver and Los Angeles. That vulnerability is rooted in the unusual resistance Trump faces among well-educated white voters: Three national polls last week each found that around 60 percent of whites holding at least a four-year college degree disapproved of his performance.

But one of the key questions for November is whether Democrats can extend that pressure into suburban Southern seats that have previously been safe for Republicans, including districts near Richmond, Charlotte, Houston, Dallas, Austin and Atlanta.

In addition, gains in white-collar suburbs will be critical to Democratic prospects in the Georgia governor's race between African-American Democrat Stacey Abrams and Republican Secretary of State Brian Kemp; the highly competitive Tennessee Senate contest between former Democratic Governor Phil Bredesen and Republican Rep. Marsha Blackburn; and the more uphill, but still competitive, challenge by Democratic Rep. Beto O'Rourke against Republican Sen. Ted Cruz in Texas.

Even if Democrats fall short in some or all of these races, a significantly improved performance in white-collar suburbs could offer them a roadmap for seriously contesting North Carolina, Georgia and perhaps even Texas against Trump in 2020 -- when African-Americans, Latinos and other minority voters, who mostly lean Democratic, will likely comprise a bigger share of the electorate than this fall.

Changing demographics not enough

Most discussion on whether Democrats can restore their tattered competitiveness in the big Southern states has focused on whether the party can increase turnout among those minority voters, who are rising as a share of the population in many Southern states. Registering and turning out more African-Americans, Latinos and other non-white voters undeniably represents an essential part of the equation for Democrats across the region, strategists in both parties agree. Abrams, in particular, has staked her campaign in Georgia largely on spurring greater turnout among minority and young voters who don't usually participate in midterm elections.

But in virtually every state in which Democrats have grown more competitive since the early 1990s, increased minority participation has been only part of the equation -- it has been necessary, but not sufficient. Whether in California, Illinois and New Jersey, which tilted toward Democrats in the 1990s, or Colorado, Virginia and (more equivocally) North Carolina, where the party strengthened its position in the 2000s, the winning Democratic formula has combined both an increase in minority participation and improved performance among college-educated white voters. In the states where Democrats have gained ground in recent decades, those two changes have offset lackluster, and an often deteriorating, performance with evangelical, rural and non-college educated white voters.

"The idea that changing demographics alone are going to carry Democrats through, particularly in a deep South state, is fanciful," says Republican pollster Whit Ayres, who has polled extensively across the South. "Changing demographics make it easier for them if they run a good campaign that appeals to whites as well."

In the South, though, Democrats have failed to even remotely approach the gains among well-educated white voters that they have posted in other areas. Democrats, for instance, had high hopes for cracking the Georgia suburbs in 2014 when they nominated the scions of two prominent local political families in the key races: Michelle Nunn (the daughter of former Senator Sam Nunn) for US Senate and Jason Carter (the grandson of former President Jimmy Carter) for governor. Yet both Nunn and Carter were crushed overall, and neither won more than 30 percent of college-educated white voters, according to exit polls. That's far below the typical Democratic performance among those voters in their best states, from Virginia through California, which generally ranges from the mid-40s through the mid-50s, according to exit polls.

Democrats were just as disappointed that year in Texas, when Wendy Davis, their gubernatorial nominee, suffered a stinging defeat to Republican Greg Abbott. Davis had emerged as a compelling national figure leading a filibuster in the State Senate against a Republican bill to restrict abortion rights, and Democrats hoped she could pry away suburban voters, especially women. But exit polls showed that Davis also carried only about three-in-ten college-educated whites -- and Abbott won comfortably.

In an interview, Davis pointed to two principal reasons Southern suburbs have remained so difficult for Democrats.

"One is that we've had a booming economy and it's the economy stupid as the saying goes, so people have felt pretty satisfied with where things are," Davis said. "We also suffer the problem of being absolutely ignored in presidential election contests which means that we haven't built the infrastructure that's necessary to communicate with a lot of the voters that are needed."

Other observers point to another systemic challenge for Democrats. The Democratic improvement in white-collar suburbs in other regions has been keyed largely by cultural affinity, since many college-educated voters, especially women, take more liberal positions on social issues from abortion and gay rights to gun control. But fewer Southern suburbanites are social liberals, probably because more of them than elsewhere are evangelical Christians or otherwise religiously traditional.

"Even among white college graduates in the South you have a much higher percentage of evangelicals," said Alan Abramowitz, a political scientist at Emory University in Atlanta. "My guess is that's the biggest factor in it: they are more conservative, but that is mainly because they are more religious."

The Trump factor

Democrats still confront all of these barriers this year. But Trump's rise has provided them a new opening. With his racially infused nationalism and belligerent personal style, Trump conspicuously lost ground across many Southern suburbs in his 2016 race against Hillary Clinton when compared to Mitt Romney's performance in the same places in 2012.

Trump's erosion in these Southern suburbs wasn't solely because of defections from well-educated white voters -- almost all of them are also growing more racially diverse. Exit polls showed Clinton stuck at about 30 percent support from college-educated white voters in Georgia and Texas and around 40 percent in North Carolina, all comparable to the Democrats' performance in the 2014 statewide races in those states. Yet in key suburbs around the major metropolitan areas, such as Gwinnett and Cobb counties outside of Atlanta, the shift away from the GOP was undeniable and ominous for Republican strategists.

"I still am stunned that the two most rock-ribbed counties in Georgia that we use to base statewide Republican wins on, Gwinnett County and Cobb County, both went for Hillary Clinton," said Ayres. "There has been so much emphasis on the blue collar counties in the Rustbelt that switched from Obama to Trump. There has been less focus on suburban counties in places like Atlanta or Houston that moved from Romney toward Clinton. Basically we've traded the smaller, slower growing more rural counties for larger fast growing suburban counties."

Elections since 2016 have offered Democrats some additional signs of progress in Southern suburbs. In last November's Virginia governor's race, Democrat Ralph Northam posted big advances not only in suburbs outside of Washington DC, which politically behave more like Northern suburbs, but also the suburban communities around Richmond, which have tilted more reliably Republican. Unusually strong performance in white-collar suburbs around Huntsville and Birmingham helped propel Democrat Doug Jones' narrow win in last December's Alabama special election for US Senate. And Democrat Jon Ossoff came close before ultimately falling to defeat Republican Karen Handel last June in a suburban Atlanta special election for a US House seat that Republican Tom Price had carried comfortably for years.

Can Democrats advance further from those suburban beachheads in 2018? Joshua Blank, manager of polling and research at the Texas Politics Project at the University of Texas, is dubious, for Texas at least.

"While this idea of hidden Democratic votes in the suburbs is alluring, the reality is Texas is an extremely conservative place," Blank said. "The future of the Democratic Party in Texas is not educated whites. I think that is more about Democratic mobilization (of minorities and young voters) than (converting) reflexive Republican whites."

A more immediate problem for Texas Democrats, he said, may be improving their performance among Latino voters, among whom Republicans have retained a significant minority of support even under Trump.

Still, the most recent statewide poll conducted by the University of Texas and the Texas Tribune in June shows some clear openings for Democrats among well-educated white voters who have previously resisted them.

While Trump carried college-plus whites in Texas over Clinton by two-to-one, according to exit polls, the June poll found those voters divided only evenly over his performance. The same poll showed O'Rourke running even with Cruz among those well-educated white voters, a much stronger performance than Democrats have notched in any recent race -- though Blank cautioned the results for that group come with a large margin of polling error. (A May Quinnipiac University poll in Texas showed Cruz still holding a substantial lead among well-educated whites.)

A test in Georgia

Of all this fall's Southern contests, the Georgia governor's race may offer the starkest pressure test of the region's suburban loyalties. Neither nominee is a particularly good fit for those voters. Though she worked pragmatically with Republicans during her years in the Georgia House of Representatives, Abrams has mostly presented herself as a liberal champion committed to transforming the state by expanding the electorate. Kemp, meanwhile, campaigned as a clone of Trump (who endorsed him) and ran polarizing ads in which he brandished a shotgun at a teenage suitor for his daughter and promised to personally "round up criminal illegals" in his pick-up truck. Tellingly, Kemp has said repeatedly that he would sign a "religious freedom" bill protecting business owners who refuse to serve gay customers; the current Republican Gov. Nathan Deal vetoed the bill for fear it would discourage business investment in the state.

Democrats see such Kemp declarations as the key to unlocking the Atlanta suburbs.

"Looking at these Southern states, the Republican coalition for governor there has been to bring together the base Republicans who are more rural, with business-minded suburbanites, but to always put business first," said Jared Leopold, communications director for the Democratic Governors Association. But now in Georgia, he adds, "If you are the business community, you have to think twice about do you get behind a Republican as you normally would, or would Kemp scare away business?"

Ayres, who polls for Handel in the Georgia suburbs, acknowledged that risk. But he predicted that Abrams will prove too liberal to exploit it -- at least if Kemp steers his campaign even modestly away from the Trump-like themes that he stressed during the GOP primary.

"If he runs a good campaign, he's got a lot more in common with those suburban voters than Abrams does," Ayres said.

The earliest public and private polling shows the two locked in a close race, with Abrams recording modest, but measurable gains from the Democrats' 2014 performance among white-collar suburbanites, particularly women. Whether or not Abrams makes history as the nation's first female African-American governor, November's results in suburban Atlanta will provide one revealing measure of whether Democrats can truly pressure Trump in 2020 in Georgia -- and perhaps elsewhere across the South.