SIOUX CITY, Iowa — The Toby Keith music blared from loudspeakers as Senator Joni Ernst, carrying an energy drink, worked a crowd of bikers in this town near the Nebraska border, shaking hands and giving out hugs.
Human-size Trump signs stood in the Harley-Davidson store parking lot beneath a bright, almost cyan blue sky, but there was a note of gloom in the voices of some supporters of Ms. Ernst, a Republican. One number was on their minds: $100 million.
That’s how much allies of her Democratic rival, the businesswoman Theresa Greenfield, are pumping into the most expensive Senate race Iowa has ever seen. Attack ads bombarding the airwaves — during college football games and conservative talk radio shows — paint the senator as a villain intent on stripping away Social Security and medical benefits for residents.
Six years after storming into office as perhaps the highest-profile member of a vaunted class that took back Republican control of the Senate, Ms. Ernst, 50, finds herself in a tough re-election race that is emblematic of her party’s struggle to keep the Senate majority with a weakened President Trump at the top of the ticket.
Ms. Ernst, who has tightly embraced the president even as his standing has fallen, has trailed Ms. Greenfield in every poll for the past month, and in a recent New York Times-Siena poll, as many Iowans had a negative view of her as those who had a positive one. The survey underscored a bitter reality for the first woman to represent Iowa in Congress: Mr. Trump’s troubles, particularly with female voters, are doing real damage to Republicans down the ballot.
The party holds a 53-to-47 advantage in the Senate, but as many as eight of its incumbents are in jeopardy of losing in hotly contested races. That includes other stars of the class of 2014 once believed to be part of a promising new generation of Republicans, including Senators Cory Gardner of Colorado, Steve Daines of Montana, Dan Sullivan of Alaska and David Perdue of Georgia.
Ms. Ernst is widely seen as a bellwether candidate, who will rise or fall with her party, and with Mr. Trump. Almost no one believes Republicans can hold onto control of the Senate if Ms. Ernst loses.
The president won Iowa by more than 9 percentage points in 2016, but he now trails or is statistically tied in state polls with former Vice President Joseph R. Biden Jr., the Democratic nominee.
While Ms. Ernst has occasionally parted ways from the president — she opposed Mr. Trump’s tariffs, for instance, and supported removing the names of Confederate military leaders from military bases — she has more often embraced him.
Keep up with Election 2020
At the Harley-Davidson event in Sioux City, Ms. Ernst — who gave a speech at the Republican National Convention this year and was once considered a potential running mate for Mr. Trump — urged her supporters to draw a “red line in the sand” against encroaching liberalism by backing the president, echoing his campaign message.
Later, speaking to reporters, Ms. Ernst said she did not believe Mr. Trump’s declining popularity in Iowa would hurt her, and argued he could still win over the suburban women who have been turning against him. But she hastened to add that she was “running my own campaign” and even suggested a number of Iowans might cross party lines to vote for both Mr. Biden and her.
“There may be issues where people will disagree with the president, but they’ll be supportive of me,” Ms. Ernst said. “So it’s really up to those Iowans to go out and make that decision, but I hope they do recognize that Iowa is where I was born and raised and Iowans are the people that I care about.”
Karen M. Kedrowski, a political-science professor at Iowa State University and the director of the Carrie Chapman Catt Center for Women and Politics, said in some parts of the state, Mr. Trump had grown “toxic,” which could affect Republicans who had not distanced themselves enough from him.
“Their fates are tied together,” Ms. Kedrowski said of Ms. Ernst and Mr. Trump. “There’s such dissatisfaction with the Trump administration, it’s spilling over to harm Republicans down-ballot.”
Ms. Ernst entered the Senate on the strength of a buzzworthy “Make ‘Em Squeal” ad, pledging to cut wasteful spending just as she had castrated pigs on her family farm. She soon became the only woman on Senator Mitch McConnell’s leadership team, and spoke out powerfully about surviving rape and domestic abuse.
But for Ms. Ernst, Ms. Greenfield presents a much steeper challenge than did Bruce Braley, a gaffe-prone former congressman whom she easily defeated by more than 8 percentage points six years ago.
With a biography that resonates with Iowans, Ms. Greenfield has proved a disciplined messenger, hammering Ms. Ernst on pocketbook issues like health care, while stressing her own background as a military mother and a “scrappy farm kid” who grew up in nearby southern Minnesota.
She wears flannel shirts in her television ads. In front of her Des Moines home sit logs of firewood Ms. Greenfield says she chops herself. “I grew up pretty dang scrappy, I got to tell you,” she likes to say.
Her farm roots were on display during a televised debate Thursday night when she correctly answered a question about the price of corn in the state, while Ms. Ernst was stumped by a follow-up about the price of soybeans.
“You grew up on a farm,” Ron Steele, a Waterloo news anchor who was a moderator of the debate, told Ms. Ernst. “You should know this.”
While Ms. Ernst was riding her motorcycle across the state last weekend, Ms. Greenfield was at the Smith family farm in Buffalo Center near Iowa’s northern border, stepping through fresh pig manure and swatting bugs away from her neck as she talked about health care.
“This is total bias, but she’s a farm girl,” said Jody Smith, 65, a farmer, explaining why she decided to support Ms. Greenfield. “I know that she has learned to work hard. She can stand up to anyone in Washington.”
Ms. Greenfield has run a careful race, presenting herself as a centrist. She does not denigrate Mr. Trump’s supporters and criticizes the Democratic Party for not focusing enough on community colleges.
She also has her own compelling story of overcoming personal tragedy. Her first husband died in a workplace accident when she was 24 and pregnant with their second child. The family survived on Social Security payments.
“Becoming a young widow changed my life,” she said. “I didn’t have any way to pay the bills.”
With a unified Democratic message coming from party leaders in Washington, Ms. Greenfield has stayed focused on the issue of health care, hitting Ms. Ernst repeatedly on her votes to repeal the Affordable Care Act and her embrace of a false claim stemming from a conspiracy theory that the coronavirus death toll was being inflated.
Political action committees connected to Senator Chuck Schumer, Democrat of New York and the minority leader, have pumped tens of millions of dollars into the race, taking aim at Ms. Ernst.
“Senator Ernst can’t be trusted on health care,” Ms. Greenfield says, calling it the top issue in the race.
Ms. Ernst has scrambled to respond. When Mr. Schumer forced a recent vote to bar the Trump administration from arguing to overturn the health care law, Ms. Ernst broke with her party to vote with the Democrats. And she apologized repeatedly for her comment questioning the coronavirus death toll.
“I am so sorry that my words may have offended you,” Ms. Ernst said during a recent debate, addressing health care workers. “You are tremendous workers. You are essential workers.”
In an attempt to salvage the seat, Republicans’ Senate campaign arm has begun running its own attack ads against Ms. Greenfield’s business record, accusing her of “shoddy workmanship” and “breach of contract” — charges the real estate developer disputes.
Ms. Ernst has also emphasized Ms. Greenfield’s failed foray into politics in 2018, when she briefly ran for Congress but ended her bid after her former campaign manager admitted to forging signatures on petition paperwork.
Some Iowans said they were turned off by the flood of negative ads against Ms. Ernst.
“The things that are being advertised against her, it makes me want to slap them,” said Denny Gergen, 69, a grain and soybean farmer from northwest Iowa and one of the bikers who turned out to support Ms. Ernst. “Yeah, I know, hey, it’s politics but this is just getting dirty.”
Ms. Ernst saw hope in Mr. Trump’s nomination of Judge Amy Coney Barrett to the Supreme Court, arguing that the confirmation fight would energize conservatives and drive them to the polls to support her. She returned to Washington from the campaign trail to participate in the hearings, where she emphasized Judge Barrett’s status as a powerful conservative woman.
But Ms. Ernst also tried to strike a moderate tone when discussing the consequences of elevating Judge Barrett to the Supreme Court, pointing out that she had once upheld a protest buffer zone around abortion clinics.
“I think the likelihood of Roe v. Wade being overturned is very minimal,” Ms. Ernst said, referring the landmark decision that established federal abortion rights. “I don’t see that happening.”
In Iowa on her motorcycle tour, Ms. Ernst issued a message that echoed that of Mr. Trump, as she warned voters of a grim future should Ms. Greenfield defeat her and Democrats take control of the Senate. A Democratic victory, she said during a stop in Des Moines, would mean a takeover of the United States by “extreme liberal interests,” “extreme environmentalists” and “extreme abortionists.”
“All of those things lead us on an ugly path towards socialism,” Ms. Ernst said.
Then she told the bikers hanging on her every word to get ready to ride. “We’re going to show the state of Iowa that we still stand behind President Trump,” she said. “We stand and are going to hold this red line in the United States Senate.”
With that, Ms. Ernst pulled her hair back into a ponytail, hopped on her Harley, revved the engine and took off, leading a small army of bikers east away from the city, American flags flying from the back of their motorcycles, many adorned in bold letters: “T-R-U-M-P.”