LAS VEGAS — Nevada airwaves have become an all-out warzone for the state’s Senate race, as the contest becomes increasingly important in the battle for control of the upper house and as conservative scions who’ve shunned Donald Trump, like the billionaire Koch brothers, unleash their resources lower on the ticket.

Outside groups, candidates and political parties spent about $25 million on TV ads for the race through the end of August, with more than $13 million coming from groups favoring Republican Rep. Joe Heck and more than $11 million backing Democrat Catherine Cortez Masto, according to an ad buying source. New ads are unveiled on a near-daily basis; the race for the open seat long held by Democratic Sen. Harry Reid is only expected to pick up after Labor Day.

“We all knew that this was probably going to be the most-watched race in the U.S. Senate,” Heck said in an interview this week. “A lot of entities are going to play hard to try to make sure this seat goes one way or another. So my advice to people is to turn your TV off until Nov. 9.”

Viewed as the only Democratic-held seat Republicans could capture during a presidential election cycle where they’re expecting considerable losses, the race is rising in strategic importance. Contests in other parts of the U.S. are becoming less competitive and the chances of Democrats securing a comfortable majority are looking dimmer.

It’s also rife with personal symbolism, pitting the Koch brothers and their network of political organizations against their bombastic archenemy Reid, who maligns their political activities at every opportunity. Reid says he’d love to keep the seat he’s held for 30 years if he didn’t have a debilitating eye injury; he’s instead serving up bare-knuckled attacks against Heck.

“In my more than 50 years around here, he is the worst, because he is the most fraudulent person I’ve ever seen try to pull the wool over your eyes,” Reid told the Reno Gazette-Journal editorial board in August.

The race has taken a noticeably bitter turn. While the springtime brought commercials introducing Cortez Masto’s big family and thanking Heck for supporting veterans, newer spots feature unsettling footage of drunken drivers swerving into oncoming traffic and accusing Cortez Masto of letting DUI offenders back on the roads early.

Major players on the Republican side include the Koch-aligned Freedom Partners, Karl Rove’s One Nation PAC and the U.S. Chamber of Commerce. Backing the Democrats are the Reid-aligned Senate Majority PAC, the AFSCME union and the League of Conservation Voters.

The battle is also raging on Spanish broadcast stations, as Republicans seek to narrow their certain losses among Hispanic voters.

As told by commercials from One Nation, Heck supports DREAMers, which defers deportation for immigrants brought to the country illegally as children, and agrees that there should be a pathway to citizenship for them. Cortez Masto’s campaign focuses on four Heck votes to defund President Barack Obama’s Deferred Action for Childhood Arrivals program, arguing he’s a threat to DREAMers.

Heck has actually voted on both sides of DACA.

“The Democrats want to draw that sharp contrast, and Heck wants to muddy the waters,” explained David Damore, a political science professor at UNLV.

Ads supporting Cortez Masto focus on Heck votes to defund Planned Parenthood, including one from a female Republican gynecologist who says she’ll vote Democratic because Heck doesn’t make sense for her patients. Others, from the group End Citizens United, point to more than $5 million in campaign contributions Heck has received from secretive sources through “dark money” groups.

Democrats have a 72,000-voter registration advantage among Nevada’s 1.3 million voters, but are less consistent in turning out than Republicans. In a state where it’s common to split the ticket, and where the race between Hillary Clinton and Donald Trump is seen as close, neither party can afford to let their voters drift to the other side in the Senate race.

Another reason for all the Senate commercials is that Nevada is a battleground, but includes just two media markets — Reno and Las Vegas. It’s cheaper to buy up ads to blanket Nevada for a statewide Senate race than it would be in other battleground states with more markets.

“This is when you want to own a TV station in Nevada,” Damore said.