A couple of decades ago, half the Democrats in the Iowa Senate represented rural areas. By the time the last session got underway, there were only two Democrats left from the mostly sparsely populated counties west of Interstate 35. Now, there are none. The inability of Iowa Democrats to compete throughout an entire half of the state is a big reason why the GOP took over the state Senate in November.

All over the country, Democrats have a similar geography problem. With an overwhelming share of their voters living within a limited number of metropolitan districts, it’s hard for them to compete in broad swaths of territory elsewhere. This handicap, which has made the U.S. House into something resembling a fortress for Republicans, is making it increasingly difficult for Democrats to win legislative chambers. “When you sit down and start counting the number of state legislative districts the Republicans have and the number of chambers they have, it’s evident that the Democrats have a structural problem that they need to overcome,” says Colorado State University political scientist Kyle Saunders.

In a red state like Texas, for example, Democratic legislators are limited to the heavily Hispanic Rio Grande Valley and just a handful of urban counties. In a purple state like North Carolina, Republicans were able to maintain their supermajorities at the legislative level, despite Democrat Roy Cooper winning the governor’s race last fall. Even in a more favorable state for Democrats such as Colorado, which Hillary Clinton carried, Republicans were able to hold onto their majority in the state Senate. The concentration of the Democratic vote in Denver and the Front Range gives Republicans a built-in advantage in the chamber. Despite a big push from Democrats to take it back, there were only two or three suburban or exurban districts where they even had a hope of picking up seats.

When Republicans won control of the Minnesota House in 2014, nearly all their victories came outside of the Twin Cities area. In November, the GOP’s strength in the rest of the state allowed the party to capture the state Senate as well. “I had one of the Minnesota leaders tell me a year ago that the House Democrats had gotten too Twin Cities-oriented,” says Bill Pound, executive director of the National Conference of State Legislatures. “That’s why they were in the minority and that was the danger to their Senate majority.”

Democrats are already hoping for an anti-incumbent wave election in 2018 that will return them to power in many states. They also like to blame GOP gerrymandering for the challenges they face in many districts outside the cities. But Iowa has a redistricting process that is scrupulously nonpartisan. In that state, rural Democrats have become not just endangered, but nearly extinct. In order to make a comeback, Democrats have to hope that voters outside of population centers will start giving their candidates more of a hearing than they have lately.