LOS ANGELES, June 5 (Xinhua) -- More than 108 million U.S. dollars has been spent in California's House races till Tuesday, the primary voting day for the mid-term elections in the Golden State, local media reported.

Citing data from the Federal Election Commission (FEC), the Los Angeles Times reported that the amount that candidates has spent so far in this election cycle is almost double what was spent at this point in the 2016 election cycle, which was 58 million U.S. dollars.

The analysts said two factors hike the cost of 2018 primary election in California, including more opportunities for candidates and unique complex top-two primary election system of the state.

Since Democrats must wrest at least 23 seats from Republican hands to seize control of the House for the second half of Trump's first term, California turned into a key battle field for the two parties as there are seven Republican seats in districts of the state won by Democratic presidential nominee Hillary Clinton in 2016 and no other state features more than three such seats.

Those districts are in suburban Orange County and San Diego, the Central Valley and northern Los Angeles County, where voters showed a stronger anti-Trump attitude after the winner of 2016 presidential election was sworn into the White House last year.

So more Democratic candidates joined in the competitive race and capitalized on the sentiment to raise huge funds. Meanwhile, more rich Democratic candidates are throwing down seven-figure amounts in hopes of capturing one of the two spots in the primary.

In CA-39, 48, and 49 districts, according to The Intercept website, 14 active Democratic candidates have raised a total of 23.73 million U.S. dollars, in which whopping 16.12 million U.S. dollars comes from the candidates' own wallets.

Under California's primary system, all candidates appear on a single primary ballot, with the top two vote-getters regardless of party advancing to the November election.

It forced the candidates to spend more money earlier than they have in previous cycles otherwise they could not emerge from the morass to fight on into November, analysts said.