This article is from the archive of our partner National Journal

Rep. Jim Costa was thought to be headed for a safe race. The California Democrat represents a district that supported President Obama by a 19-point margin in 2012 -- and while Costa had had scares in previous midterms, 2014 appeared to be in the bag. And indeed, Costa did keep his seat, but only after squeaking through one of the tightest races of the entire cycle, a vote count so close that it took 15 days for him to be officially declared the victor over Republican candidate and dairy farmer Johnny Tacherra.

So what happened? Costa nearly fell victim to a radically different midterm electorate. The total number of ballots cast in the race dropped 40 percent from 2012 to 2014, according to data compiled by Dave Wasserman of The Cook Political Report.

Costa's story is a microcosm of the turnout problem that plagued Democrats up and down the 2014 ticket, but it's particularly troubling for the party because Costa's district is among the most heavily Latino in the country.

Turnout also plunged in other heavily Latino districts, such as the one where Democratic Rep. Jerry McNerney narrowly won reelection in a California district Obama won by 18 points in 2012. There, turnout was down nearly 47 percent between 2012 and 2014. In the state's 31st District, Rep.-elect Pete Aguilar, who was the heavy favorite in his race, won by just over 3,000 votes after a 52 percent drop-off of voters from 2012 to 2014. And Rep. Lois Capps, whose district Obama won by 11 points in 2012, won by fewer than 8,000 votes after a 32 percent drop-off.

All of this sets up an uncomfortable conundrum for Democrats: If this is purely a cyclical problem and the presidential race brings Latinos back to the ballot boxes, then their recent woes have little bearing on 2016. But that's a big "if," and one that no Democratic presidential candidate will be willing to take as a given.

Ben Tulchin, who runs a Democratic campaign research firm in California, said the lack of enthusiasm among Latino voters raises questions about Democratic presidential hopefuls and whether the party can motivate voters during midterms. "How do we regain momentum with Latinos?" Tulchin said. "Hillary [Clinton] has had some success there. Can she regain that spark? And how do we sustain that through 2018 and beyond? The demographics are on our side, but if it only matters every four years, well, elections are every two years."

The presidential nominee on the Republican side is equally important, said Allan Hoffenblum, publisher of the California Target Book. If it is someone like former Florida Gov. Jeb Bush, who speaks Spanish and whose wife and son are Latino, "that could change the equation" for California Republicans, Hoffenblum said.

All in all, California Democrats held their ground, narrowly winning these races and others, increasing their majority in the state's congressional delegation by one seat. But in other heavily Latino districts in the West, Democrats weren't so fortunate.

In Nevada, Rep. Steven Horsford lost in perhaps the biggest House upset in the country after a 48 percent drop-off in votes. Texas Democratic Rep. Pete Gallego, though he was considered an easier target than the others, also lost a close race after a 41 percent drop-off. The common denominator among these upsets: Each of the districts' populations are at least 29 percent Latino.

Both parties seemed to be caught off guard by how close some of the California House races were, even though Costa and McNerney faced similar scenarios in 2010, and though California's dull gubernatorial race meant that the state's least reliable voters would have little motivation to go to the polls.

But it shouldn't have been such a surprise, said Dan Schnur, director of the University of Southern California's Jesse M. Unruh Institute of Politics and an unsuccessful independent candidate for California secretary of State.

"It was very clear that the Democratic base was not going to turn out in very large numbers this year," Schnur said.

Even so, it's hard to say whether it would have made a difference if either party had known that these Democrats were vulnerable. Hoffenblum said it probably benefited the Republican challengers in these districts that their party didn't get involved, because it would have prompted Democratic outside groups to do the same. In fact, the biggest mistake Republicans made was releasing a poll showing Capps's challenger with a 1-point lead, Hoffenblum said. The Democratic Congressional Campaign Committee reacted by spending $99,000 on last-minute radio ads.

"I thought if they had kept their mouths shut they might have won that thing," Hoffenblum said.

It can also be difficult to predict the sentiment in a district that has vastly different electorates depending on the year. Costa's, McNerney's, and Horsford's districts, among others with large Latino populations, tend to be overwhelmingly Democratic during presidential years -- not the kind of districts the National Republican Congressional Committee could justify investing in, or where the DCCC would bother to get involved.

As defensive as Democrats were this cycle, they could be just as aggressive in 2016. Costa, McNerney, Aguilar, and Capps will all have a much more liberal electorate, and the party's biggest targets -- freshman Reps. Scott Peters, Ami Bera, Julia Brownley, and Raul Ruiz -- will all have another year under their belts, Hoffenblum said.

At the same time, Republicans whom Democrats unsuccessfully targeted this cycle will also have a different set of voters. Republican Rep. David Valadao, whose district is 72 percent Latino and supported Obama by an 11-point margin in 2012, faced a challenge from Amanda Renteria, who was the first Latina chief of staff in Senate history while working for Michigan Sen. Debbie Stabenow. Valadao won reelection easily this year, but after a cycle of campaigning and higher name recognition, Renteria could have a shot in two years if she runs again, Tulchin said.

Republican Rep. Jeff Denham may also be a major target for Democrats, Tulchin said. Denham won reelection by 12 points over Michael Eggman, a farmer and beekeeper who raised far less money. Considering the major drop-off in turnout this year, Denham's 12-point win is not particularly impressive going into 2016, Tulchin said.

"You can win some of these," Tulchin said. "I'm not going to say easily, but the math is better. Valadao, he won by a decent margin this time; same with Denham, but with higher turnout, especially Latinos voting, it's a different dynamic."

This article is from the archive of our partner National Journal.

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