To get a sense of a congressional district's political leanings, it's generally a good idea to start with a metric called the Partisan Voter Index, or PVI, which was created 20 years ago by the Cook Political Report. Districts that lean slightly towards Democrats might have a PVI of D+2 or D+3. Districts that are safely in Republican hands might show a PVI of R+10 or greater.
In general, competitive districts are seen as having a +5 advantage or less for either party, but out of 435 U.S. House districts, eight of them have a special designation: they're exactly "even." Neither party has any advantage at all, making these eight districts the most competitive in the nation.
One of them, California's 10th, was represented by Rep. Jeff Denham (R) of California, whose "blue" state is getting "bluer" by the day. Naturally, the five-term incumbent faced an uphill climb this year, and he didn't quite make it: Denham lost to Rep.-elect Josh Harder (D) by about three points.
The outgoing congressman spoke to the Sacramento Bee about the race, and pointed to a variety of factors.
Denham said his own race, as well as those of other Republicans, were faced with trouble this year from a Democratic operation well-positioned to turn out voters, unprecedented Democratic fundraising and new California voter laws designed to register more younger voters. [...]
Also hurting Denham was the big Latino vote. Ethnic breakdowns of the overall vote haven't been tabulated. But early returns on Latino voter turnout indicated their participation was about the same as in presidential election years for the first time in a midterm election.
Denham pointed to other factors. He said Republicans were hurt by California's motor voter law, which encourages 16- and 17-year-olds to pre-register to vote as they get their driver's licenses so they will be automatically registered at 18, and same-day registration, which allowed people to register up until Election Day and cast a provisional ballot.
That's not quite how I'd recommend looking at the results. In Denham's case, the California Republican voted with Donald Trump's position the overwhelming majority of the time, including siding with his party on unpopular measures such as health care repeal and tax breaks for the wealthy.
In a district with an "even" PVI, one might expect a lawmaker like Denham to have one of Capitol Hill's most centrist voting records. He didn't.