WASHINGTON — The U.S. Senate contest between Republican Sen. Kelly Ayotte and Democratic Gov. Maggie Hassan is set to become the most costly political race in New Hampshire history, with spending reaching $100 million — maybe more — by Election Day.

But despite all the money, the race hasn’t budged from a dead heat, with most polls showing Ayotte and Hassan running neck-and-neck.

Combined, the candidates themselves have spent less than $10 million on television advertising, with Hassan being the first to launch a negative ad earlier this week. But that’s a drop in the bucket compared to the tens of millions of dollars outside groups are spending to blanket the airwaves with overwhelmingly negative ads. The Center for Responsive Politics, a non-partisan group that tracks money in politics, records $25 million in outside spending so far, but that excludes a handful of groups that are subject to different disclosure laws. In reality, the spending is closer to double that and only poised to increase.

Senate Majority PAC, a group aimed at delivering Senate control to Democrats, has reserved $6.3 million in advertising time for the fall on top of the $7.3 million it has already spent, while the Republican-affiliated Senate Leadership Fund is pouring nearly $16 million into the contest through a group called Granite State Solutions. The Democratic Senatorial Campaign Committee and National Republican Senatorial Committee plans to spend at least $8 million and $6.8 million, respectively, according to spokespeople. One Nation, another prominent GOP group, has spent at least $8 million on television, according to its spokesman. None of those totals count money spent on digital advertising or get-out-the-vote efforts.

The massive amount of spending is a reflection of the importance both campaigns are placing on the match-up, deemed one of the truest toss-ups in the nation.

“This has and continues to be a central battleground for us,” said Lauren Passalacqua, a spokeswoman for the Democratic Senatorial Campaign Committee.

Dante Scala, a political science professor at the University of New Hampshire, said the fact that the race remains tight even as polling averages show Democrat Hillary Clinton with a lead over Republican Donald Trump in New Hampshire may be evidence that ads on behalf of Ayotte in particular are working.

“It’s keeping the race in contention even while the presidential race is appearing to be very unbalanced toward the Democrats,” he said.

It’s difficult to calculate exactly how much has been spent, because groups are subject to different disclosure laws based on how they are organized. But a picture of spending can be captured by tracking Federal Election Commission reports and public ad buys. Calculations from both Republican and Democratic groups show the race coming close to $100 million in outside spending by Election Day, far exceeding the total spent in the 2014 race between Democratic Sen. Jeanne Shaheen and GOP challenger Scott Brown.

Both Hassan and Ayotte are painting themselves as victims of a bombardment of outside “special interest” cash. But each has her share of outside backers. Hassan has said she supports campaign finance reform, but she’s still seeing millions in outside money spent on her behalf. She’s been supported on television by Planned Parenthood, the pro-gun control group Americans for Responsible Solutions and a handful of other groups.

Her campaign maintains Hassan’s outside backers hold views that match New Hampshire values, while Ayotte’s do not. In her own ads, Hassan focuses on state-based accomplishments like balancing the budget — a requirement by law — and lowering tuition at community colleges.

“(Hassan) is fighting for issues and priorities that are the issues and priorities of the people of New Hampshire,” campaign spokesman Aaron Jacobs said. “Kelly Ayotte’s priorities are the Washington special interest priorities.”

Ayotte, for her part, has won support from the U.S. Chamber of Commerce and the Judicial Crisis Network, among other groups. In her campaign ads, Ayotte paints herself as an independent voice who will buck her own party. Meanwhile, her allies portray Hassan as a poor manager of the state, particularly when it comes to New Hampshire’s heroin and opioid addiction crisis.

Ayotte’s campaign has yet to launch any negative ads of its own, but it did air an ad showing the state’s junior senator on a baseball field batting to symbolize how she’s swatting away what the campaign calls negative and misleading attacks.

“We’ve obviously had to run an aggressive campaign in terms of responding to some of those ads,” spokeswoman Liz Johnson said.