Democrats' route to the House majority runs through California more than any other state. Nationally, they need to win 23 GOP seats to win the chamber, and in California alone, there are seven Republican incumbents sitting in seats Hillary Clinton carried in 2016. Four of those seven seats include at least a piece of Orange County, which in 2016 voted for a Democrat for president for the first time since 1936.

But California's unusual "top two" primary — combined with an explosion in the number of Democratic candidates running in 2018 — could throw a wrench into Democrats' fall plans in Minority Leader Nancy Pelosi's backyard.

Under California's rules, all candidates in all 53 districts run on the same primary ballot and the top two finishers, regardless of party, advance to the general election. In a few GOP-held seats, there are so many Democrats running that they risk splitting enough votes to allow two Republicans — and zero Democrats—to advance to November. That would be a nightmare scenario for Democrats in otherwise terrific pickup opportunities.

And whereas some other states' laws allow "sore losers" to run write-in or independent candidacies on the November ballot after losing a primary, there is no such recourse in California for a party that catastrophically gets shut out of the top two.

Although a shutout isn't especially likely in any district, it's a legitimate fear for Democrats. Despite President Trump's unpopularity in the state and the news that registered Republicans just fell behind "no party preference" voters into third place on the state's rolls, statistics on vote-by-mail ballots show Republicans, white voters and older voters returning primary ballots at significantly higher rates than Democrats, non-white voters and young voters.

Originally, Democrats' greatest shutout fear was in California's 48th CD, where GOP Rep. Dana Rohrabacher faces an intra-party challenge from former ally and Orange County GOP Chair Scott Baugh. But with several late polls showing Baugh falling back, some Democrats are more concerned about the 39th CD, where GOP Rep. Ed Royce is retiring and a GOP drive to recall state Sen. Josh Newman over his gas tax vote is priming GOP turnout.

While Democrats are worried about getting "locked out" in at least one House district, Republicans are more concerned about getting "locked out" in races for Senate and governor. If that were to happen, Republicans might have a difficult time motivating their voters to cast ballots in the fall, hurting their chances in crucial House races. Fortunately for them, Trump-endorsed GOP attorney John Cox looks likely to advance in the governor's race.

Here's a quick rundown of ten of the top California House primaries to watch on Tuesday:

GOP Seats Where Democrats Risk a "Lock Out":

CA-39: This Orange County-based seat is probably where Democrats face the greatest risk of two Republicans advancing. Popular longtime GOP Rep. Ed Royce, the chair of the House Foreign Affairs Committee, is retiring from a district Trump lost by eight points. Republican Young Kim, a former Royce staffer and Assemblywoman, is the favorite to finish first in Tuesday's primary. But the race for the other runoff slot is a free-for-all.

The Democratic front-runners are a pair of millionaires, pro-impeachment insurance businessman Andy Thorburn and DCCC-endorsed Navy veteran/lottery winner Gil Cisneros. But two Republicans, former state Sen. Bob Huff and Orange County Supervisor Shawn Nelson, are also competing for the second runoff slot. And two other Democrats, pediatrician Mai Khanh Tran and attorney Sam Jammal, could further split Democratic votes.

A big reason why Democrats are nervous here is turnout: 22 percent of 39th CD Republicans have returned ballots versus 17 percent of Democratic voters (unlike in the 48th and 49th CDs, where Democrats have turnout out at higher rates). That could be because a GOP drive to recall Democratic state Sen. Josh Newman over his vote in favor of a gas tax increase has energized GOP voters in an almost entirely-overlapping state senate seat.

CA-48: Veteran GOP Rep. Dana Rohrabacher, who has faced criticism for his embrace of Russian president Vladimir Putin and Julian Assange, is likely to finish first in Tuesday's primary for this coastal Orange County seat. But the race for the second November slot is a three-way contest between two Democrats, real estate businessman Harley Rouda and stem cell researcher Hans Keirstead, and Republican attorney Scott Baugh.

Democrats feel more confident about their position here than they did several weeks ago. Not only have the DCCC's attacks on Baugh seem to have kept his support down, but Rouda appears to have gained late momentum amid questions surrounding Keirstead's exit from a UC-Irvine lab and Keirstead's fumbling of a question about his failure to vote in past Democratic primaries at a recent debate. Now, a Democratic shutout here would be a surprise.

CA-49: GOP Rep. Darrell Issa, who won by just 1,621 votes in 2016, is retiring after nine terms from this coastal San Diego County district. But his retirement may not have been a gift to Democrats. That's because his exit prompted four credible Republicans to join a field of four credible Democrats running here — and now, theoretically, any combination of parties is possible on the November ballot.

Fortunately for Democrats, late polls show GOP voters consolidating behind former Dana Point Mayor Diane Harkey, with three other Republicans (Assemblyman Rocky Chavez, San Diego County Supervisor Kristin Gaspar and financial adviser Bryan Marryott) fading. That should allow one of the three leading Democrats (2016 nominee Doug Applegate, attorney Mike Levin or former State Department contractor Sara Jacobs) to advance.

Other Top Democratic Opportunities:

CA-10: GOP Rep. Jeff Denham will finish first in this Modesto seat, but the race for the second November slot still looks like a jump ball between three Democrats: 2014/2016 nominee Michael Eggman, venture capitalist Josh Harder and EMILY's List-endorsed former Riverbank Mayor Virginia Madueno. There is a second Republican, former Turlock Councilman Ted Howze, on the ballot, but Central Valley observers consider a lockout unlikely.

CA-21: GOP Rep. David Valadao only faces one opponent on the primary ballot — Democratic engineer/businessman T.J. Cox — so there's no question who will advance to November. But the primary vote is worth watching to see whether the political environment has changed here since Valadao took 57 percent in 2016 while Hillary Clinton carried the district by 14 points. Holding Valadao under 60 percent in the primary might be encouraging for Cox.

CA-25: GOP Rep. Steve Knight will finish first, but two Democrats, attorney/2016 nominee Bryan Caforio and non-profit executive Katie Hill, are battling for the second slot. Recently the race has turned nasty, with Caforio attacking the conditions at homeless shelters Hill's non-profit ran. Republicans successfully attacked Caforio as a slick downtown LA carpetbagger last time, so a Hill nomination would probably boost Democrats' fall odds.

CA-45: GOP Rep. Mimi Walters sits in a district Hillary Clinton won 49 percent to 44 percent, but her favorable numbers are still strong back home. A pair of Democratic UC-Irvine law professors, Katie Porter and Dave Min, are running neck-and-neck for the second slot. Porter, a protege of Sen. Elizabeth Warren, is running as the progressive activist. Min, a Korean-American hewing closer to the center, might be the stronger match against Walters in November.

Democratic Long Shots to Watch:

CA-04: GOP Rep. Tom McClintock sits in a suburban Sacramento district Trump won 53 percent to 39 percent, but he's a career politician who's underperformed before. Two Democratic women with national security profiles, former USAID official Jessica Morse and former State Department official Regina Bateson, are competing to advance. Both have outraised McClintock in recent quarters, but Morse looks like the favorite. It would take a big wave to flip this in the fall.

CA-22: GOP Rep. Devin Nunes's White House-friendly handling of the Russia investigation as chair of the Intelligence Committee has helped Democratic prosecutor Andrew Janz raise $1.4 million, but it's also helped Nunes raise almost $2 million. Trump won the district 51 percent to 42 percent and Nunes took 67 percent in 2016. If Nunes is under 55 percent of the primary vote on Tuesday, Democrats may take a closer look, but it's still a long shot.

CA-50: GOP Rep. Duncan Hunter is under a federal grand jury investigation for misuse of campaign funds, which is the only reason this San Diego district Trump won by 15 points is worth watching. Democrats' best hope for the fall is likely a Hunter match up against former Navy SEAL Josh Butner, but he faces competition for the second November slot from Democratic former Labor Department official Ammar Campa-Najjar and GOP El Cajon Mayor Bill Wells.