Marsha Blackburn and Phil Bredesen CNBC; Getty Images

KNOXVILLE, Tenn. — The battle to succeed retiring Republican Sen. Bob Corker in Tennessee has seen a surge in outside groups trying to influence the hotly contested election. And voters are sick of it. "I hate that they ran everything so negative" said one voter, Brent Larson. "Vicious," said another, Barbara Tallent. "One of the ugliest ones in my lifetime." In a ride-along Saturday with Americans for Prosperity, an outside group funded primarily by billionaire industrialist Charles Koch and one of the organizations spending millions in the state, CNBC spoke with almost a dozen voters in Knoxville, the third most populated city in Tennessee. Every voter who spoke on the record said they normally vote for Republican candidates. That should be a good sign for GOP Rep. Marsha Blackburn, one of the candidates vying to replace Corker. But rival Phil Bredesen, a Democratic former governor, has been making inroads with Republicans. The Koch network as a whole has endorsed Blackburn in the race. Yet the Americans for Prosperity efforts Saturday were focused not on directly pushing the GOP candidate, but more on people who have been inconsistent in turning out to vote. In 2016, the state was ranked 48th among the 50 states that year in voter turnout, according to the nonpartisan data website FairVote.org. Meanwhile, the group's affiliated super PAC, Americans for Prosperity Action, has been pushing some of the same ads that voters appear to despise. The committee has spent $2.1 million in support of Blackburn and $433,000 against Bredesen, according to the nonpartisan Center for Responsive Politics.

The Bredesen battle

A recent ad calls on voters to turn against Bredesen due to a record, the group said, that reflects a push for higher state taxes when he was governor and spending $9 million in taxpayer money on revamping the governor's mansion. Bredesen has also seen help from Democrats outside the state looking to influence the election. The Senate Majority PAC, a super PAC affiliated with Democratic Senate Minority Leader Chuck Schumer, has spent more than $3 million against Blackburn. Billionaire and former New York City Mayor Michael Bloomberg recently hosted a fundraiser for Bredesen in Manhattan. Outside groups have totaled more than $30 million in spending on both sides, making it the most expensive campaign in Tennessee history. Tim Phillips, president of Americans for Prosperity, told CNBC in an interview that he believes the ads serve as a tool to educate voters on the resumes of both candidates. "We think it's helpful to have voters on both sides to have a thorough hearing about the differences between the different candidates," Phillips said. "It's an important moment for the country and a great deal is at stake," he added. Representatives for Blackburn and Bredesen did not return interview requests.

What voters are saying

Voters are still divided over whom to vote for, even as ads inundate the state. For Bredesen, the good news is that there appears to be a growing contingent of Republicans who are willing to vote for him, which would signify a remarkable turnaround for Democrats. The GOP has represented Tennessee in the Senate for the past 24 years, and Trump won the state by 26 points over Hillary Clinton in the 2016 election. The nonpartisan Cook Political Report labels the Tennessee U.S. Senate race as a toss-up. The Real Clear Politics polling average shows Blackburn up by 6 points. Stan Harrell, who spoke to CNBC from outside his home, said he votes for Republicans, but this year he's likely to make a different choice because the former governor appears to be more genuine then Blackburn. "I felt like he voices his own opinions and takes an nonobjective view out there," Harrell said. "In Blackburn's case, one of the things that concerns me is her support for Trump and she doesn't appear to have her own objective view." Tallent, a registered Republican who voted for Trump but will back Bredesen in 2018, pointed to his work as a governor and mayor of Nashville as reasons to vote for Bredesen. "I thought he did some great things for Nashville. He gave a boon to Nashville's economy and did great things for that city," she said. One of his crowning achievements as mayor was recruiting two professional sports teams to come to Nashville, which is nearly 200 miles west of Knoxville. In the 1990s, Bredesen helped bring the National Football League's Houston Oilers to the Music City, where they are known as the Tennessee Titans. Bredesen also was mayor when the city landed a National Hockey League expansion team, the Nashville Predators.

The Trump factor