BILLINGS, Mont. (AP) — The Republican Party's hold on the U.S. Senate could be decided by a tight race in Montana where Democratic Gov. Steve Bullock was trying to unseat first-term GOP Sen. Steve Daines.

The heavyweight battle has been one of the most closely watched Senate races in the U.S., with political groups and the campaigns pouring more than $100 million into the contest.

That's a record-shattering figure for the sparsely populated state that dwarfed spending in much larger battlegrounds and subjected residents of Big Sky Country to a barrage of advertisements virtually any time they turned on their computer, television or radio or opened their mailbox.

With results still coming in, Daines was watching from a Bozeman hotel room with his family, spokesperson Julia Doyle said.

“We're very optimistic we are in a good place," she said of the early returns.

In Helena with his family, Bullock thanked his supporters in a brief video message shortly before polls closed.

“It’s been a challenging eight months for sure, but our campaign has accomplished so much,” he said.

Montana's other congressional seat is also expected to be a tight contest with Republican Matt Rosendale seeking to fend off a challenge from former Democratic state lawmaker Kathleen Williams to keep Montana’s lone U.S. House seat in GOP hands in a streak that dates to 1997.

Williams lost her bid for the seat in 2018 but came closer than any Democrat in 20 years to winning. Polls suggest she has an even better chance this time around. Victory would make her Montana’s first congresswoman since Jeannette Rankin won the seat in 1940.

“I entered this race to make sure we have a true independent voice in our lone seat in Congress," Williams said in a video recorded on election night.

Her campaign focused on her career in environmental conservation and the promise of improved health care and public lands protection.

Rosendale, a former real estate developer, hitched his campaign to President Donald Trump and stuck to the Republican party line, promising to protect the Second Amendment and end illegal immigration.

He said Tuesday night he would withhold any celebration until all votes were tallied. Rosendale added he was pleased that in the final weeks of the campaign he was able to travel across the state and meet with voters despite the worsening pandemic.

“To go back out and be able to meet with folks again was quite enjoyable. This last week we were really crushing it,” Rosendale said.

Trump's handling of the coronavirus and the economy has loomed over both races, but the president didn't come to Montana to stump for either candidate.

After Trump easily won Montana in 2016, Daines emerged as one of his loyalists and stuck with the president through the coronavirus pandemic, a faltering economy and civil unrest stirred by police killings of people of color.

Daines, a former business executive, was serving his first term in Congress six years ago when Democratic Sen. Max Baucus resigned to become ambassador to China. That opened the seat for the first time since 1978 and Bullock appointed then-Lt. Gov. John Walsh to fill it. Democrats hoped the ploy would give Walsh an advantage but he dropped out late in the race amid a plagiarism scandal, clearing the way for a Daines victory.

Republican voter Debbie Rowe, an insurance agent from Billings, said Daines had been a “great senator” during his first term. She dismissed Democratic allegations that Daines would sell off or transfer public lands.

“When you know the person you can tell when it’s not necessarily the truth,” Rowe said.

Bullock was a private attorney and then served two terms as Montana attorney general before his 2012 election to governor. While seeking the 2020 Democratic nomination for president he adamantly rejected a Senate run, and moved to the left on issues including guns, which Daines would later use against him.

Under pressure from Democratic party leaders including former President Barack Obama, Bullock threw his hat in the ring in March, sticking with the themes that drove his presidential bid: expanded health care access and a pledge to bring a Montana ethos of practical problem solving to Washington.

Within a week of his entering the Senate race, the arrival of the coronavirus upended the state and Bullock was in the limelight as he issued orders closing schools and restaurants followed by a general stay-at-home directive.

Stay-at-home mother Holly Hayes of Laurel, one of the first in line to vote in Yellowstone County on Tuesday morning, said she was impressed with the Democratic governor’s education policies, his handling of the coronavirus and his expansion of Medicaid.

“Steve Bullock has done really well as governor for two terms,” she said. “He had the schools really in mind and all the citizens of Montana in mind.”

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Find AP’s full election coverage at APNews.com/Election2020.