Think all-party primaries tell you something instructive about how one candidate might fare against another in the general election? Think again.
Certainly, it's an intriguing idea. States that pit office-seekers from opposing parties against one another on the ballot should, logically, test how those candidates might do in November.
But past all-party primaries in California — like the one Tuesday night — have proved a poor predictor of what's going to happen on Election Day, and that's because low turnout skews what might otherwise be a pretty powerful indicator.
Almost across the board in congressional primaries, these early votes serve as a high-water mark for Republicans and a baseline for Democrats, whose performance improved by 7 percentage points on average between the primary and general elections in 2012.
That means if Democratic House candidates do poorly in the Golden State on Tuesday, political oddsmakers should resist the urge to downgrade their chances for the fall, as some forecasters did after the 2012 primary.
Consider the data. In 2012, California's first year running the new top-two primary, all but two House Democratic candidates improved his or her party's vote share between June and November. Only five Republicans managed the same feat. (This analysis totals the vote share of all Democratic and Republican candidates in the primaries, and it leaves out districts that didn't pit a Democrat against a Republican in the general.)
That continued a pattern seen when California used an all-party primary setup in 1998 and 2000. There were 83 House races in those two years where one party improved its standing between the primary and the general. Democrats did it 66 times, compared to 17 for Republicans. Democrats did 4 points better on average in the 1998 general election and 2 points better in the 2000 general election.
Overall, across 1998, 2000, and 2012, Democrats only failed to improve their performance between primary and general in 18 percent of California congressional races.
Certainly, these data points underscore a concern for Democrats too: the party's reliance on inconsistent voters to boost its candidates to victory.
Many of the biggest jumps in 2012 came in heavily Hispanic districts. Democratic Rep. Mark Takano, who represents Riverside, won less than 46 percent of the primary vote and then romped to victory with 59 percent in the general election. Rep. Raul Ruiz, the Democrat from the neighboring Palm Springs-based district, won 42 percent in the primary and then unseated Republican Mary Bono Mack with 53 percent in November.
Part of the reason is that Hispanic turnout craters in California's primaries. According to figures collected by the Public Policy Institute of California, the Latino share of the vote was more than 7 percentage points higher in November 2012 than June 2012, a repeat of what happened in 2008. In 2010, a midterm year — when nonwhite turnout is typically lower — the Latino vote share still jumped over 5 points for the general election that fall. The same PPIC report also shows that young voters, another key Democratic constituency, are underrepresented in the state's primaries. Meanwhile, the California voter data firm Political Data Inc. has found Hispanic early-voting rates this year lagging past years' rates for primaries.
Both parties have pickup opportunities scattered throughout California's 53 congressional districts. Republicans are running hard at four freshman Democrats in particular: Reps. Ruiz, Ami Bera, Julia Brownley, and Scott Peters. Meanwhile, Democrats have focused on two Republican-held seats, one belonging to Rep. Gary Miller, who is ending his congressional career, and the other belonging to Rep. David Valadao, who is just starting his.
Democrats rightly worry about their heavy reliance on less-than-reliable voters as they try to protect House and Senate seats this fall. But the situation isn't as bad as the head-to-head nature of California's primaries makes it look.
Zach Cohen contributed to this article