Lawrence C. Levy is executive dean of the National Center for Suburban Studies at Hofstra University and a former columnist and editorial writer at Newsday who has covered seven presidential elections. The views expressed in this commentary are his own.

(CNN) If the Democrats are to regain power in Washington, they'll need to start in the places that for decades have provided the decisive vote in presidential and congressional elections: the nation's suburbs.

Suburbanites are the largest yet least predictable voting bloc. Not only did they open the White House doors to Donald Trump , they also delivered the decisive margins in US House and Senate races that protected his party's congressional majority -- the key to realizing a conservative Republican agenda.

The April 18 special election in Georgia's 6th Congressional District will be a litmus test of where the parties stand with swing suburbanites, in an area that reflects the demographic and social changes that have made many suburbs increasingly competitive.

Like suburbs through the nation, the 6th District for years was so reliably Republican that the last incumbent, Tom Price -- who gave up the seat to be Secretary of Health and Human Services -- once ran without opposition in 2004 and often won with 70% of the vote or more. The district produced Newt Gingrich, who led a short-lived "Republican Revolution" as a backlash to Bill Clinton in 1994 that nonetheless was a precursor to the party's current successes.

What the suburbs giveth one year, even one political season, its moderate, independent swing voters can take away in another. Republicans and Democrats have learned this the hard way over the years, often after winning big in a presidential year. And, as we're seeing in Georgia, well before the 2018 midterm elections, Republicans will receive a series of signs of what Trump's historically low approval rating -- and a sudden surge in Democratic activism -- may mean for their futures.

That's not only a matter of presaging election victories, but gauging public support -- or opposition -- to sweeping, fundamental change. Both politics and policy collided last week in the GOP's disastrously divisive drive to accomplish the rare feat of rolling back major social legislation, by "repealing and replacing" Obamacare.

The effort, which allowed the party and President to be portrayed as both incompetent and cruel, damaged the Republican brand with independents -- many of them suburbanites -- and even some core supporters. And it leaves Democrats even more energized.

A win in Delaware

In late February, Democrats won a pivotal state Senate race in Delaware , riding a wave of anti-Trump anger to retain control of the upper chamber. Democrats throughout the country were especially excited by the extraordinary turnout in this heavily suburban district -- twice as much as usual for an "off-season" special election -- as well as by a margin of victory that was 10 points higher than last November's.

Still, some Republicans may not categorize the Delaware win as Democratic progress, because this district was drawn for a Democrat to win

But last November's results in Georgia's 6th District offered the GOP signs of trouble in this once Republican paradise. Trump barely won the district against Hillary Clinton, performing relatively poorly -- as he did in most suburbs throughout the country -- among more prosperous, well-educated suburban voters who were less susceptible to the Republican's populist appeal. Trump also did poorly among minority voters in the district, including Asians, who were turned off by his anti-immigration mantra. He just managed to carry the district, as he did in many states, with a strong showing among blue collar whites in outer-ring suburban communities.

Although Price won re-election by more than ten points, it was one of his worst showings. It's numbers like those, as well as unprecedented antipathy among parts of the electorate for the new commander in chief, that give Democrats hope for a seat they haven't held in decades.

However, this remains a strong GOP district by enrollment and tradition. If Democrats want to win, they will have to remain unified if a member of their party makes it into the likely runoff. Fracture along Republican ideological lines, which pit conservatives against those who are more moderate, could dilute the party's voting strength. This could be a real canary in the coal mine for GOP problems in 2018 and beyond.

A runoff likely in Georgia

The Democrats have coalesced around a single candidate, Jon Ossoff , who is raising millions of dollars nationally and locally. His campaign says it has thousands of volunteers in the field. Republicans have a number of strong candidates likely to compete in the primary, so it's probable that April 18th won't be the end of the story; a runoff between the Democrat and the best-performing Republican may well follow.

The 6th isn't the GOP's only vulnerable district. Nationally, 23 Republican members of Congress won districts that Hillary Clinton also won. The majority of them are suburban. About 40 congressional races in all, out of 435, were considered competitive . Almost all were entirely or substantially suburban.

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And they offer opportunity. Unlike cities and rural communities, almost unshakably blue and red respectively, suburbs are shades of purple, with voters going back and forth among the parties.

A prime reason these places tend be more competitive is their political, ideological and demographic diversity. A surge of Democratic-voting new immigrants and other minorities are changing the demographic and political face of many suburbs that once were nearly all white and heavily Republican. These trends are accelerating.

But it's not only "demographic destiny" that poses challenges for the GOP in swing districts. The more moderate suburban mindset reflects homeowners who see federal, state and local government not as an enemy or savior, but as a partner in providing good public schools, safe streets and clean, green parks. They'll pay high taxes and support expensive programs if they feel they're getting value for their dollars. They'll revolt if they don't.

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Hofstra's National Suburban Poll has consistently shown support among suburbanites for government-assisted health care coverage, public education and environmental protection -- desires that run counter to the Republican agenda, as reflected in the Obamacare repeal debacle, several cabinet appointments and Presidential pronouncements. Dismantling departments dealing with education and the environment are not high on the suburban agenda.

Overall, suburbanites of all ages like things to be calm and predictable -- unlike what they're seeing in news reports from Washington. That's why off-year local elections in November -- races for county executives, town supervisors and village mayors -- will offer another litmus test on where the nation is headed politically.