Control of the Iowa Senate is up for grabs this fall. Democrats currently have a 26-24 majority, a meager margin Republicans are eager to erase. Given the circumstances, you’d expect both parties to press hard to win every available seat. But that’s not the case. Half the Senate seats are up in November, but in nine of the 25 contests, one of the major parties hasn’t bothered fielding a candidate.

That’s more common than you might think. When filing deadlines had passed in the first 27 states this year, one party or the other had failed to run candidates in nearly half the legislative seats -- 45 percent, according to Ballotpedia, an online politics site that tracks races and ballot initiatives. Nearly all incumbents can rest easy in Georgia, because 80 percent of the races there will be uncontested.

In recent years, it’s been common for a third to 40 percent of state legislative seats to lack major party competition. It’s even worse during primary seasons, meaning legislators win re-election simply by showing up. In the four states that held legislative elections last year, 56 percent of the races went uncontested in the fall.

There are a number of reasons for this. For one thing, redistricting has left most legislative seats lopsided in favor of one party or the other. Why bother running if you figure that more than half the voters are already against you? Also, potential challengers know that incumbents have huge advantages in terms of resources. So they choose to wait until the right moment to run. The percentage of seats that are actively contested goes up in years when one party or the other believes the wind will be at its back -- 2008 for Democrats, for example, or 2010 for Republicans.

People who recruit candidates -- legislative leaders and party officials -- also pick their battles. They are much more concerned with getting quality candidates in place in districts that look winnable than they are in making sure there’s a warm body occupying every slot on the ballot.

That may be a mistake, says Steven Rogers, a political scientist at St. Louis University who studies uncontested elections. You can’t win a race if you don’t put someone on the ballot. In most races in most years that doesn’t matter, but politics is a game full of surprises. An especially weak presidential candidate, for instance, can open up a lot of potential races at the legislative level. “If Democrats don’t have candidates on the ballot in states where they don’t like Donald Trump, they may be leaving seats on the table,” Rogers says.

In Iowa, officials with both parties say they are perfectly content with their recruitment strategy, even if they have already conceded 36 percent of the races. “In that state, we feel great about candidate recruitment,” says David Griggs, national political director for the Democratic Legislative Campaign Committee. He’s encouraged by “the quality of the candidates on the ballot.”