Non-stop ads and robocalls: welcome to America's costliest election

Republicans are seeking re-election in nine of the 10 most expensive House races. The tenth is an open Republican seat. Each recorded spending of more than $20 million.

Four of the costliest races are in California. Contests in Washington, Pennsylvania, Colorado, Florida, New York and Minnesota round out the list.

California's 25th district is one of the most competitive. Republicans have held the seat since 1992. But the past decade has seen an influx of newcomers priced out of the Los Angeles housing market, and now nearly 40 percent of the district's 720,000 residents are Hispanic.

Democrats set their sights on taking the district after 2016 presidential candidate Hillary Clinton won it by seven points and a Republican, Representative Steve Knight, clinched another term by six points.

This year, Knight, 51, has been outraised and outspent by Democratic challenger Katie Hill, who previously ran a non-profit for the homeless and is seeking office for the first time.

Hill, 31, raised $7.3 million by mid-October, compared to the $2.4 million raised by Knight, according to FEC data. Hill has out-spent Knight $5.9 million to $2 million, with nearly three quarters of her money going to advertising.

With money from outside groups factored in, more than $18 million has been spent on Hill's behalf, compared to about $8 million for Knight, Reuters found.

Independence USA PAC, almost entirely funded by former New York mayor Michael Bloomberg, has spent more than $4.5 million in ads supporting Hill.

The money helped the political newcomer blanket the district with her liberal message supporting a path to universal healthcare, an assault weapons ban and gay-and-lesbian rights.

That could prove a double-edged sword. District voter Burke said she had leaned toward Knight. Then the avalanche of Hill ads and mailers annoyed Burke so much it cemented her decision to cast her ballot for him.