National divide in presidential elections Percentage-point margin R+30 R+27 Rural and small towns More GOP R+1 Suburban/ midsize metro 0 More Dem D+30 D+32 Big urban ’04 ’08 ’12 ’16

The suburbs and midsize metro areas are poised to make an impact Tuesday. Because they have the largest numbers of voters, even a shift that looks unremarkable in percentage points can yield a decisive margin in votes.

Big-city suburbs and similar midsize metros together make up the biggest bloc in more than half of the competitive congressional districts on Tuesday’s ballots. And in another handful of districts that comprise all three kinds of areas — Big urban, Suburban/midsize metro, and Rural and small towns — the suburbs have enough voters to swing the balance.

That gives suburbs an outsize role in deciding the 73 competitive House districts, all but four held by Republicans. Democrats would need a net gain of 23 seats to win control of the chamber.