Today is the most important day of 2018 so far for Democrats' chances in the House

That's because the massive (and massively important) state of California votes today, with polls closing at 11 p.m. Eastern time. And unlike in election cycles past, where California -- despite its size -- was a competitive wasteland due to a redistricting process in 2011 that sought to firm up incumbents of both parties, there are a surfeit of competitive contests this year.

CNN rates nine Republican-held seats as competitive or potentially competitive, as does Cook Political Report, a non-partisan handicapping site. Inside Elections, another campaign handicapper, carries eight California Republican seats on its competitive race list.

That may not seem like a huge number. But when you consider that Democrats need to net just 23 seats nationwide to recapture the House majority they lost in the 2010 election, then the importance of California begins to come into much clearer focus. If Democrats can run the table of competitive seats in California this fall, they would be almost halfway to the 23 pickups they need. In a slightly less optimistic scenario for Democrats, a pickup of six seats gets them one-quarter of the way to the majority -- in one state alone!

Which brings me back to today -- and California's very odd "jungle primary" system. Under the 2010 law, which was approved by ballot initiative by California voters eight years ago this month, candidates run on the same ballot. The two top vote-getters -- regardless of party -- advance to the general election. The goal of the jungle primary was to mitigate party polarization -- the theory being that with all the candidates on one ballot, there would be far less incentive for Democrats to run way to the left and Republicans to run way to the right. The ignored moderate voter would be essential again.

Like all seemingly fool-proof solutions, the jungle primary has produced unanticipated problems -- especially for Democrats in 2018. Because of the strong anti-Trump fervor within the party, Democratic candidates have flooded races up and down the ballot in California. With so many Democratic candidates running, party strategists worry that the vote will be badly splintered -- not only opening up the possibility that two Republicans make it into the top two, but also in some races allowing an underfunded and perhaps weaker Democratic candidate to eke through as the party's lone hope in the fall.

That possibility has created a fascinating dynamic in the closing weeks of the California campaign as Democratic outside groups have been forced to make triple-bank-shot spending decisions -- attacking some of their own candidates in the hopes of ensuring a single strong Democrat emerges for voters today.

Writes CNN's Maeve Reston:

"Campaign strategists and outside groups have resorted to an elaborate series of chess moves in which they have battered third, fourth and even fifth-tier candidates to drive down voter support for those contenders in the hopes of avoiding a splintering of the vote."

If that complex series of strategic calculations backfires -- or just plain doesn't work -- it could spell disaster for Democrats in the fall. Of the 25 House seats currently held by Republicans that Hillary Clinton carried in the 2016 election, seven(!) of them are in California. Those districts range from Rep. Dana Rohrbacher's (R) 48th District that Clinton won by 2 to Rep. David Valadao's (R) 21st District where Clinton won by 16.

In short: What happens in California tonight matters. A lot.