New Jersey Rep. Rodney Frelinghuysen’s retirement announcement this week wasn’t a game changer in the battle for control of the House of Representatives. Even before his announcement, Frelinghuysen, a Republican, was in danger of losing a re-election bid in the 11th Congressional District, which President Trump won by a mere 1 percentage point in 2016. With Frelinghuysen out of the race, Democrats and Republicans are expected to fight it out over the now-open seat.

But even if the Frelinghuysen news isn’t earthshaking, it’s a reminder that whether Democrats manage to win the House in 2018 could come down to how many seats they pick up in the five most populous states that Hillary Clinton won in 2016: California, Illinois, New Jersey, New York and Virginia.

The vast majority of Republican House members don’t hail from these states, of course. In fact, of the 241 seats Republicans controlled after the 2016 elections, just 42 (17 percent) were from these five states. That’s not surprising — this isn’t GOP territory; Clinton carried all these states by at least 5 percentage points, and House and presidential voting are increasingly connected.

But while California, Illinois, New Jersey, New York and Virginia account for only a small percentage of Republican-held seats overall, they are home to a disproportionate share of vulnerable Republicans. According to the Cook Political Report, these five states are home to 38 percent of all the Republican-held seats that are truly in play in 2018.

Well-populated Clinton states may be the GOP’s demise Number of vulnerable* GOP House seats, according to the Cook Political Report, that are in the most populous states won by Hillary Clinton in 2016 House REPUBLICAN Seats States No. Vulnerable TOTAL percent vulnerable New Jersey 4 5 80% California 8 14 57 Illinois 4 7 57 Virginia 4 7 57 New York 5 9 56 Total 25 42 60 *Vulnerable seats are those rated as “lean Democratic,” “toss-up,” “lean Republican” and “likely Republican.” Source: Cook Political report

California has the largest number of vulnerable Republican House seats with eight (out of 14 GOP-held seats in the state). But New Jersey — Frelinghuysen’s state — has a higher proportion of flippable GOP seats (four of five). Even if we take “likely Republican” seats off the table and look at just those seats in the lean and toss-up categories, a majority of New Jersey Republicans still make the list. This includes Frelinghuysen’s seat, which is rated as a toss-up.

The most interesting thing about these states, though, is the total number of Republican seats that are rated as at least somewhat vulnerable. If you add them all up, a total of 25 Republican seats in these five populous Clinton states could flip to the Democrats. That’s one more seat than Democrats need to gain a majority. In other words, they could take back the House without flipping a single seat in a state that Trump came close to winning in 2016.

Now, Democrats are probably not going to win a House majority based solely on heavily populated blue states. The competitive districts in these Clinton states aren’t all alike. Some are well-educated, like Virginia’s 10th Congressional District. Others are best described as working-class, like New York’s 22nd District. Some are whiter than the nation as a whole, like New Jersey’s 11th; others are majority non-white, like California’s 39th. The national political environment — what turnout looks like in November, and which groups Democrats over- or underperform with — will therefore manifest itself differently in each of these districts.

The chances are that Democrats are going to lose at least a few of these blue-state seats, while also flipping some seats in states that Trump won in 2016, such as Arizona, Florida and especially Pennsylvania, where six Republican seats are vulnerable.

It does seem pretty clear at this point, however, that the 2018 midterm elections are going to be fought on very different turf than the special elections that dominated the 2017 landscape, or even the 2016 election. Instead of national reporters rushing to red states like Alabama, Georgia, Kansas and Montana, or parachuting into the Rust Belt, they’ll be setting up camp in well-populated blue states.