No longer overwhelmingly white and prosperous, many of these demographically diverse places -- where more than half the nation's people live -- are struggling with economic, social and other problems more commonly associated with deteriorating cities. And their votes are up for grabs: Unlike the largely "blue" urban and "red" rural voters, whose political loyalties are obvious and consistent, their suburban counterparts are unpredictable -- except in their willingness to "swing" between parties.
So it's a mystery why this year's corps of presidential candidates, Republican and Democratic alike, aren't talking about the suburbs or at least framing major national issues in a way that would resonate with the millions of moderate suburbanites.
Take the last Democratic debate, which aired last week on CNN. Not long after the opening applause, both Hillary Clinton and Bernie Sanders made impassioned appeals to -- and for -- the residents of Flint, Michigan, a small city whose water was contaminated by lead and other poisons in the public water supply.
Both Clinton and Sanders made the same salient point -- that Flint is not the only community threatened by water-borne poisons and overwhelmed by the costs, if not politics, of replacing aging or corrupted delivery systems. And that the federal and state governments have been AWOL in helping municipalities not only monitor dangerous pollution but pay for such massive public works.
When it comes to taking on the costly and complicated challenges of such large-scale public works projects, however, there's a big difference between major urban centers and smaller suburban municipalities.
When the suburb is among the increasingly poor, predominantly minority enclaves with deteriorating finances and infrastructure and a dearth of professionally trained leaders, it's not surprising people in these places feel abandoned.
And that's what made the Flint-related exchanges between Clinton and Sanders a missed opportunity.
When Clinton and Sanders reached for examples of the many other communities facing pollution and public works challenges, did they cite one of the thousands of politically "purple" suburban counties currently at risk? No, they offered up Cleveland and Detroit, respectively.
It's no secret that urban centers are suffering, and it's no secret how they will vote (Democratic). But it's in the suburbs that the war for the White House will be won: The majority of the voters nationwide are in the suburbs -- especially in the 10 or so swing states where suburbanites essentially will decide who wins the presidency.
So why miss this prime opportunity to woo the voters that can really make a difference?
Clinton's suburban aversion may have cost her in Michigan where, aside from a photo-opish foray to Flint, she focused almost exclusively on Detroit's Wayne County.
Clearly, she didn't pay special attention to the Motor City suburbs of Oakland and Macomb counties where she underperformed polling expectations (as well as in Detroit itself).
And the tactic is surprising because the suburbs have been a strong suit against Sanders throughout the primary season in winning a healthy mix of white moderates and the growing number of suburban minorities.
In some ways, Clinton abandoned her history.
Arguably, the suburbs are a Clinton family thing: In 1992, Bill Clinton almost single-handedly made the suburbs safe for the Democratic Party by projecting a more moderate image than its last several presidential candidates.
Hillary Clinton won two Senate elections in New York with the same approach. She once proposed legislation that would review federal rules and revenue streams for disparate impact on needy suburbs.
The conventional wisdom is that during primary season, candidates appeal to party loyalists who often are the antithesis of swing voters. Then, if they win, the candidates hope they have time during the fall to pivot toward moderates, especially in the suburbs. But ignoring these voters in the primary will only make the task harder.
Democrats are hardly alone in banishing the S-word.
Republicans once dominated many of these places between city and country until demographic and ideological forces shifted the fulcrum in the past few decades. But even with the need for political remediation, the GOP candidates -- even Marco Rubio who actually had a "suburban strategy" -- have not spoken directly to suburbanites in terms of specific problems and policies.
Trump has done relatively poorly in the suburbs, even in states he has won.
If he turns out to be the Republican nominee, he will have a lot of work to do in the general election to have any chance of winning over moderate suburbanites. They tend to shy away from extremist campaigns from either side. They've alternately punished Republicans and Democrats when they stray too far from the middle.
The generic Republican message that the federal government should be doing less may not play well among suburban mayors and supervisors who need big-buck help. The same goes for all those suburban baby boomers, the age group that tends to vote in the highest numbers, who are feeling the pressure of being the "sandwich generation" responsible for their aging octo- and nono-generian parents and struggling millennial children.
Who will help the boomers with the costs of nursing care and college loans if not government?
Suburbanites are not anti-tax per se -- many vote directly for enormously high local taxes to support schools, for instance. And they don't see government as the enemy -- they rely on it to preserve the investment in their homes and business they own at higher rates than urban or rural counterparts.
Even though they may complain about taxes, they will spend lavishly, especially on education and the environment, if they believe they are receiving value.
Now the suburbs need a lot of help with everything from crumbling infrastructure and deteriorating downtowns to increasing air and water pollution. But help won't arrive any time soon unless the rest of the country looks past the myths of wealth and wellness. And so far, in the presidential campaigns, attention has not been paid.