The big spending doesn’t just signal that each party sees the Senate seat as winnable. It’s also a measure of intensity on both sides to prevail in Wisconsin after Donald J. Trump shocked Democrats in 2016 by being the first Republican presidential nominee to carry the state since 1984. National Democrats are bent on winning it back in 2020 — and getting Ms. Baldwin re-elected is a crucial step toward that goal.
The fight may become the most expensive Wisconsin Senate race ever: An analysis by the state Democratic Party found that nearly $10 million in advertising had already aired or been purchased by outside groups against Ms. Baldwin or in favor of Mr. Nicholson. (Mr. Nicholson’s camp put the number at nearly $9 million.) At least another $3.7 million in advertising is underway sponsored by outside groups in favor of Ms. Baldwin.
The advertising by political action committees like Restoration PAC and Americas PAC — both heavily financed by the hard-right industrialist Richard Uihlein — has propelled the first-time candidacy of Mr. Nicholson, and underscores the influential role that outside conservative PACs play in this politically polarized state. Organizations funded by Mr. Uihlein and the billionaire Koch brothers have devoted millions to assuring continued Republican control of the Senate, and many conservatives view Ms. Baldwin, 56, as a beatable first-term senator who symbolizes the extreme left wing of the Democratic Party.
“Tammy Baldwin is very vulnerable,” said Brad Courtney, the state’s Republican chairman, calling her one of the Senate’s most liberal members. “There’s going to be lots of money coming into Wisconsin.”
Even in this rural area of small farms, nearly two hours from Milwaukee, it’s hard to avoid the drumbeat of ads, which began well before the traditional start of campaign season.
“I hear a lot of stuff on the radio,” said Gary Buchholz, a soil technician who was part of the crowd at J & J Fireball Lanes, a local bowling alley, and plans to vote for Ms. Baldwin. “I don’t like the money that comes in from out of state, huge amounts of money trying to influence Wisconsin elections.”
Partly to counter the advertising, an energized Democratic base is organizing early, determined to retain the Senate seat held by their party since 1957, when William Proxmire was elected to the unexpired term of Joseph McCarthy, who had died in office. Yet Wisconsin has become a Republican stronghold: The state not only voted for Mr. Trump, but has also become a laboratory for conservative policy ideas under its two-term governor, Scott Walker, and the Republican-controlled legislature.
In November, Ms. Baldwin is expected to face either Mr. Nicholson or Leah Vukmir, a conservative state senator favored by the state’s Republican establishment. Ms. Vukmir, 59, has also benefited from political action committee spending, with a $935,000 ad buy by a group called Wisconsin Next PAC, funded partly by the Beloit businesswoman Diane Hendricks. Ms. Vukmir has also received scores of endorsements from state Republicans, including the support of Reince Priebus, a Wisconsinite who served as White House chief of staff.
Democrats privately expressed concern that the negative advertising has whittled away at Ms. Baldwin’s support. A March poll by Marquette Law School revealed that her approval rating was a mere 37 percent.
Fearing a reprise of the 2016 Wisconsin Senate race — when outside groups targeted the Democratic candidate, Russ Feingold, who lost even though he had held a lead two months before the election in a Marquette poll over the incumbent Republican, Senator Ron Johnson — Ms. Baldwin’s campaign has already dug into her formidable campaign chest. Her campaign said it planned to have 60 field organizers in place statewide by the end of next month.
Ms. Baldwin’s campaign also said it had released television advertising earlier than any incumbent Senate Democrat nationally, including one last week emphasizing her support for Wisconsin’s cheese industry. Dairy farms have been a staple of rural areas of Wisconsin like Portage, the county seat of Columbia County, one of 23 Wisconsin counties carried by Mr. Trump that President Barack Obama had won in 2012. The industry has been hard-hit statewide, particularly in Columbia County.
“We’re in a crisis situation, losing a farm a day,” said Sarah Lloyd, whose family milks 350 cows and who supports Ms. Baldwin, partly because of her efforts to bolster a milk price federal insurance program.
Ms. Lloyd, who previously ran for Congress, was among about 100 Democrats who braved a harsh April snowstorm to attend the party’s annual county dinner at a motel in this town of about 11,000 residents. Despite the weather, organizers said the meeting was the second-biggest turnout of Democrats ever in Columbia County.
“Hillary took Wisconsin for granted,” said Ms. Lloyd, referring to the 2016 Democratic nominee. “We’re not going to let that happen again.”
The next day, with snow still falling, a hardy group of about 150 Republicans turned out for a Lincoln-Reagan Day luncheon in Mequon, an affluent and reliably Republican suburb of Milwaukee.
Addressing the crowd at the River Club of Mequon, in a ballroom with a panoramic golf course view, State Representative Jim Ott shared his concerns about a “blue wave” in November, describing how “40 angry Democrats” had taken over his normally staid town hall meeting.
The results of two recent Wisconsin elections in which outside spending was a factor have added to Republican worries.
In a January special election upset, a local medical examiner, a Democrat, easily defeated a Republican state legislator for a State Senate seat held by Republicans for 17 years. In that race, the Republican got help from radio and digital advertising by Americans for Prosperity, a Koch-funded group. Then, in April, in a statewide Supreme Court election, a liberal judge from Milwaukee County defeated a county circuit judge backed by conservatives. The winner in that race received a boost from digital ads paid for by the National Democratic Redistricting Committee, an organization headed by former Attorney General Eric H. Holder Jr.
In many ways, the dynamics in Wisconsin mirror what’s happening nationally within the Republican Party, with deeply conservative newcomers allying with outside donors to challenge more traditional Republicans.
The insurgent candidacy of Mr. Nicholson — a Wisconsin native and Bronze Star recipient who has earned more than $1 million in the past two years as a consultant — appears to have been shaped in large part by the money of Mr. Uihlein, the founder of a shipping and industrial supply company.
Long a power broker in Illinois and Wisconsin political circles, Mr. Uihlein has taken a more aggressive national approach this year, spending $20 million to back conservatives in races across the country. In March of last year, a new Uihlein-backed PAC, Solutions for Wisconsin, announced that Mr. Uihlein had contributed $2 million to support a Senate run by Mr. Nicholson. In all, according to a recent Democratic Party analysis, spending by Uihlein-funded groups in favor of Mr. Nicholson and against Ms. Baldwin exceeds $5.4 million. Mr. Uihlein did not respond to a request for an interview regarding the Wisconsin Senate race.
Mr. Nicholson, a telegenic 40-year-old who always seems to have a fresh haircut, said he had begun exploring the idea of a Senate run well before Mr. Trump’s election, seeking support both inside the state and from national donors.
“The coalition is very impressive,” Mr. Nicholson said, reeling off a list of six groups supporting him, including four groups that have received large donations from Mr. Uihlein. “We have a lot of groups stepping in to say, ‘We’re going to help you take back that seat.’”
Mr. Nicholson’s political metamorphosis from Democrat to Republican has attracted thinly veiled criticism from some within the party who question his sincerity. Ms. Vukmir, who worked as a registered nurse for many years, told the crowd in Mequon, “I’m 100 percent pro-life and I have always been 100 percent pro-life.”
It was an apparent reference to Mr. Nicholson’s evolved position on abortion. In 2000, while president of the national College Democrats, Mr. Nicholson spoke at the Democratic National Convention in Los Angeles, emphasizing his support for a woman’s right to choose, among other liberal causes.
Mr. Nicholson has given various interviews about his transformation from Democrat to Republican, saying that he departed the 2000 convention disillusioned with the party.
Perhaps illustrating the political polarization of this state, Mr. Nicholson says he hasn’t spoken with his Democratic parents in more than a year, blaming their estrangement on his political choices. “No doubt, obviously for them, it created a lot of frustration and disagreement that I feel the way I do and they made a decision that I think is unfortunate,” Mr. Nicholson said.
His parents are donors to Ms. Baldwin’s campaign.
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