(CNN)Red pockets. Romneyland. Blue-collar blues.
Those labels describe the three groups of seats in the House of Representatives that will likely determine control of the chamber in November's midterm election.
Virtually all analysts in both parties agree that the epicenter of vulnerability for the House Republican majority is in what could be called red pockets: These are the predominantly white-collar suburban seats the GOP still holds in big metropolitan areas that are otherwise solidly Democratic. Those include places such as Orange County, California; New Jersey; Miami; and suburbs outside of New York City, Chicago, Denver, Philadelphia and Minneapolis.
But even if Democrats make big gains in those districts, there aren't quite enough of them to provide the party with the net gain of 24 seats it needs to recapture the House. That means House Democrats would also need to make inroads into the two other large groups of vulnerable GOP seats.
One is what I call Romneyland: white-collar suburban seats in purple and even red states where Mitt Romney, the 2012 Republican presidential nominee, almost universally performed better than Donald Trump did in 2016.
The third are the "blue-collar blues": mostly blue-collar, non-urban seats in blue states, where Trump almost without exception improved on Romney's performance.
More distantly, the Democrats are also eyeing a few Republican-held blue-collar seats in purple and Republican-leaning states. But control of the House will likely turn on how the battle between the parties turns in the red pockets, Romneyland and the blue-collar blues.
The red pockets will likely be the most reliable source of gains for Democrats because they compound two sources of Republican vulnerability: They are white-collar districts in areas with large numbers of Democrats, who appear motivated to turn out at higher-than-usual levels next fall.
The big question is how far Democrats can reach into places where only one of those advantages is present: white-collar seats in traditionally Republican-leaning areas, or seats in Democratic states that are more rural and blue-collar.
This battlefield reflects the long-term trends that have seen Democrats demonstrate increasing strength up and down the ballot in diverse, heavily college-educated, major metropolitan areas -- even in Republican-leaning states. In parallel, Republicans have established dominant control over preponderantly white non-metro and blue-collar areas, even in otherwise Democratic-leaning states.
All of these trends have accelerated under Trump. Amid improving attitudes toward the Republican tax bill, several recent polls have shown a small but measurable uptick in President Trump's job approval rating and a narrowing into the mid-single digits of the previously double-digit advantage Democrats have held in the "generic" ballot test of preferences for 2018.
But even with those shifts, polls still consistently find a deeply polarized electorate. Trump and the GOP retain solid support among white men without college degrees, if slightly less than they attracted in 2016. But they are facing intense resistance from younger and non-white voters -- especially African-Americans -- and much lower numbers than usual among college-educated whites, especially, but not exclusively, women.
White women without a college education, whose support was critical to Trump's 2016 victory, loom as a wild card: Polls find they have cooled on him, but Democrats still face many obstacles with them.
These diverging demographic attitudes shape the geography of the 2018 battlefield.
The clearest opportunity for Democrats is the relatively few remaining Republican-held districts in blue metro areas with large populations of college-educated whites, and in many cases substantial minority and youth populations as well. These are places crowded with voters who tilt toward liberal positions on social issues and recoil from Trump's volatile persona, particularly the way he talks about race.
The renewed visibility of gun control issues after the horrific Parkland, Florida, massacre could provide Democrats another lever in these districts, since the Republicans in them have almost universally voted with the National Rifle Association to loosen gun regulations in recent years.
These "red pockets" include the four seats Republicans control in Orange County -- the districts held by Mimi Walters and Dana Rohrabacher and the open seats that will be vacated by Darrell Issa and Ed Royce -- as well as their sole remaining seat in Los Angeles County, held by Steve Knight.
Others that fit this description include the seats in the western Chicago suburbs held by Republican Peter Roskam and in the eastern Denver suburbs held by Mike Coffman; the three suburban Philadelphia seats held by Ryan Costello, Mike Fitzpatrick and Pat Meehan (who has announced he will not seek re-election amid a sex scandal); the northern Virginia seat held by Barbara Comstock; two open seats in New Jersey as well as the one defended by Rep. Leonard Lance; Lee Zeldin's seat in eastern Long Island; the suburban Minneapolis seats now held by Jason Lewis and Erik Paulsen; the Seattle-area seat that Dave Reichert is leaving; as well as the Miami-area seat being vacated by Ileana Ros-Lehtinen and the nearby seat held by Rep. Carlos Curbelo.
Though Romney carried many of these seats -- often narrowly -- in 2012, Hillary Clinton won all of those listed above in 2016 except for the seats held by Lewis and Fitzpatrick, which Trump won by eyelash margins, and Zeldin's, which Trump won more comfortably. These resemble the places where Democrats showed the most dramatic gains in 2017, for instance in their sweep of legislative seats and the huge margins they generated in the governor's race in northern Virginia.
Compounding the GOP's vulnerability, the new congressional map the Pennsylvania Supreme Court issued Monday, after earlier ruling that the current district lines represented an impermissible gerrymander, strengthened the Democrats' hand in all three suburban Philadelphia seats.
For Republicans, the key in these booming districts will be whether the good economy helps them recapture voters recoiling from Trump's personal behavior. One complication is these blue-state upper-middle-class suburbs are among the most likely losers from the GOP tax plan, which limits the deductibility of mortgage interest and state and local taxes. Democrats are highly unlikely to win back the House without maximizing their gains in the red pockets.
The next bucket of seats is demographically similar to the red pockets but politically distinct because they are in metro areas that lean much more reliably toward the GOP.
I call this group of seats Romneyland because they are filled with voters who resemble Romney demographically and ideologically: professionals and corporate middle managers who want a president who will shrink government and even pursue a center-right social agenda, but also exude professionalism and decorum.
Romney won virtually every seat in this category in 2012. In 2016, Trump lost ground relative to Romney in almost all of them, though the residual Republican strength was great enough that he still carried many, albeit often narrowly.
The districts in this bucket include the Omaha-area seat held by Don Bacon; the seats in suburban Houston and outside Dallas held by John Culberson and Pete Sessions, respectively; the two suburban Atlanta seats held by Karen Handel and Rob Woodall; David Young's seat outside Des Moines; the Tucson-area seat Martha McSally is vacating to run for the Senate from Arizona; the Lexington, Kentucky-area seat held by Andy Barr; the seats outside Detroit that Dave Trott is vacating and Mike Bishop is defending; and Kevin Yoder's seat in suburban Kansas City, Kansas.
These seats are not immune from the forces threatening the Republicans in the red pockets: Handel, for instance, only narrowly survived last June's special election in Georgia, though her predecessor Tom Price had carried over 60% of the vote there as recently as 2016.
But as Handel's slim victory over Democrat Jon Ossoff showed, Republicans have more of a cushion in these places than in the red pockets. That's partly because more of the white-collar whites in them are social conservatives than their counterparts in the Democratic-leaning metro areas.
The third key test for Democrats is the districts I call "blue-collar blues." These are the blue-collar, exurban, small town and rural seats in states that generally lean Democratic.
These include Republican seats held by John Faso, John Katko and Claudia Tenney in upstate New York; Mike Bost, Rodney Davis and Randy Hultgren in downstate Illinois; the northeast Iowa seat of Rod Blum; Bruce Poliquin's northern Maine district; and the Central Valley, California, seats of Jeff Denham and David Valadao.
These seats present an especially revealing test for Democrats. Former President Barack Obama carried almost all of them at least once and many of them have elected Democratic House members in the recent past. But House Democrats were routed in these places in the 2010 and 2014 midterm elections under Obama, and almost all of these districts turned further toward Trump in 2016.
The 2017 results in Virginia and Alabama showed Democrats almost completely failing to crack the GOP's hold on blue-collar and rural voters. But some Democrats argue that terrain is much tougher for the party in the South than in the Northeastern and Midwestern states where these competitive House seats are concentrated.
Democrats see an opening in polling, such as the 2017 average of Gallup's daily approval ratings for Trump, that shows a significant erosion in his support across the Rust Belt among working-class white women, even as he remains very strong among blue-collar white men. Converting that female disillusionment with Trump into votes for Democratic congressional candidates is likely the key to seriously contesting the "blue-collar blue" seats.
One early test will be March's special election in the heavily blue-collar southwestern Pennsylvania district that Republican Rep. Tim Murphy has vacated: Democrat Conor Lamb, a former Marine, is running competitively against Republican state Rep. Rick Saccone in a district Trump carried by nearly 20 percentage points.
Beyond the top-tier targets in the red pockets, there's a spirited behind-the-scenes debate among Democratic strategists about which of the remaining two buckets of seats represents the party's best opportunity in 2018.
Some focus most on winning back working-class voters in blue states, who have a recent history of voting for the party but are more favorable to Trump. Others see more opportunity to convert white-collar suburbanites in red states, who have a longer history of voting Republican but are also more disaffected from the President.
To recapture the House, Democrats will likely need to do a considerable amount of both.
CORRECTION: This story has been updated to note that President Donald Trump won the New York congressional district of Republican Rep. Lee Zeldin, in eastern Long Island.