In the run-up to the 2018 midterm elections, Democrats have invested heavily in Orange County congressional races they think could be instrumental in taking back the U.S. House. In the June 5 primary, Democratic candidates made gains in the more reliably Republican areas of the county, putting four congressional seats in play this fall.

Democrats are competing in the 39th, 45th, 48th and 49th districts in Orange County.

Shrinking GOP margins

Not only did Democrats avoid a lockout in key House races, they bolstered their overall vote share from previous election years.

Republicans maintained the majority of the vote in three of last Tuesday's races, but saw their margin of victory shrink from previous midterm primary elections. And that could spell trouble in the fall.

The number of registered Republicans in California had already reached an all-time low in May. Primary election turnout — especially in non-presidential years — is notoriously low among Democrats and independents in California. Historically, Republican turnout remains more consistent.

Party vote share in primary elections (2012-2018)

2018 Primary results as of 12:00 p.m. on June 11. Final vote share in 2018 is likely to lean more Democratic.

Democrats have been making consistent advances

Hillary Clinton's loss in the 2016 presidential election did not overshadow one encouraging victory for Democrats: Clinton was the first Democrat to win the Orange County in a presidential election since 1936.

Her victory in the county provided a sign that Richard Nixon's birthplace was changing, and the county where "all the good Republicans go to die" had electoral promise for Democrats.

In the mid-1990s, Republicans outnumbered Democrats 52% to 32%. By 2012, the year Mitt Romney beat Barack Obama in Orange County by 7 percentage points, registered Republicans had fallen to 41%.

Democratic strongholds like the one in the 47th District in Santa Ana remain. But blue is seeping into wealthy suburban precincts in Huntington Beach and Costa Mesa.

Margin of total vote share in Congressional races

2014 primary 2018 primary

Battleground O.C.: Where Democrats are hoping to win this fall

District 39 Open seat currently held by Ed Royce

District voter registration:

34% Democratic | 35% Republican | 27% No Party Preference

2014 primary Party Vote share Republican 71% Democrat 29% 2018 primary Party Vote share Republican 54% Democrat 44%

On the ballot in November: Gil Cisneros (D) | Young Kim (R)

Republican Young Kim, a businesswoman and former state legislator, and Gil Cisneros, a Democrat and Navy veteran turned millionaire, will compete for this seat left open by Royce. The Democratic share of the vote sharply rose in this district, but Republicans also have the biggest share here of any other GOP-held district in Orange County. Minority voters could be key in this district: Cisneros has long had connections to the Congressional Hispanic Caucus, and Kim would become the first Korean American woman elected to Congress.

Margin of total vote share in Congressional races

2014 primary 2018 primary

This post will be updated with more analysis from District 39 once precinct-level data from San Bernardino County are made available.

District 45 Incumbent: Mimi Walters

District voter registration:

31% Democratic | 38% Republican | 27% No Party Preference

2014 primary Party Vote share Republican 69% Democrat 28% 2018 primary Party Vote share Republican 53% Democrat 43%

On the ballot in November: Katie Porter (D) | Mimi Walters (R)

Though this district is noticeably far less deep red than it was four years ago, Walters is still in a pretty good position. Her Democratic opponent, UC Irvine law professor Katie Porter, has cast herself as staunchly progressive, which could make it tough to flip this district unless a flood of Democratic and no party preference voters show up in November.

Margin of total vote share in Congressional races

2014 primary 2018 primary

District 48 Incumbent: Dana Rohrabacher

District voter registration:

30% Democratic | 40% Republican | 25% No Party Preference

2014 primary Party Vote share Republican 68% Democrat 32% 2018 primary Party Vote share Republican 53% Democrat 46%

On the ballot in November: Dana Rohrabacher (R)

Rohrabacher's Democratic opponent has not been announced yet; votes from the primary are still being tallied in the district.

While Rohrabacher’s higher profile has done him no favors, there’s a lot more blue in his district than there was four years ago, upping Democrats’ chances of flipping this district. It’ll probably come down to how well either local businessman Harley Rouda or stem cell scientist Hans Keirstead is able to consolidate Democratic votes, and persuade no party preference voters to go to the polls in November.

Margin of total vote share in Congressional races

2014 primary 2018 primary

District 49 Open seat currently held by Darrell Issa

District voter registration:

31% Democratic | 36% Republican | 27% No Party Preference

2014 primary Party Vote share Republican 62% Democrat 38% 2018 primary Party Vote share Republican 48% Democrat 51%

On the ballot in November: Mike Levin (D) | Diane Harkey (R)

GOP candidate Diane Harkey is a state tax board chairwoman who won first place in the primary and has better name recognition than her Democratic opponent, Mike Levin.

Democrats have to feel pretty good in the race to represent the 49th Congressional District. Four Democratic candidates captured 51% of the vote from a crowded field of eight Republicans and four third-party candidates.

On the surface, the results suggest Democrats have a strong chance of flipping this seat, replacing the retiring Issa, who won reelection in 2016 by about 1,600 votes.

Margin of total vote share in Congressional races