Mr. Ryan, meanwhile, has been coy about his intentions. After passing a landmark tax overhaul that fulfilled a career-long dream, he has not yet announced his candidacy for re-election, saying he will make a decision after consulting with his wife in the spring. That has prompted speculation that he will retire, which his aides have dismissed as nonsense.
In another election year, Mr. Bryce and the rest might be little more than a nuisance for Mr. Ryan, who has nearly $10 million in his campaign chest and is one of the most powerful men in Washington. Mr. Nehlen, strategists in both parties agree, is clearly headed for another trouncing.
But in an election cycle when Democrats have scored victories in places like Virginia, southwestern Pennsylvania and even deep-red Alabama, some analysts say it would be a mistake for the speaker to coast, especially against Mr. Bryce, who has captured the attention of Washington and Hollywood and had raised $4.75 million by the end of March, according to campaign officials.
History shows it is not impossible to knock off a congressional leader. In 1994, when Republicans swept the House during the first midterm election of Bill Clinton’s presidency, they also swept the Democratic speaker, Thomas S. Foley, out of office.
“Randy Bryce is a more formidable candidate on the resource side and the notoriety side than any opponent Ryan has faced,” said Charles Franklin, director of the Marquette Law School Poll, run out of Milwaukee. “And if it’s a wave election, who knows?”
At 48, Mr. Ryan has represented this corner of southeastern Wisconsin for nearly two decades. People here still remember him as a fresh-faced young congressman with a compelling personal story. His father died when he was 16, and he saved his Social Security benefits to pay for college. He is viewed as a nice guy and a family man.
But Janesville, a blue-collar community that was hit hard when the local General Motors plant shut down in 2009, has never been solid Ryan territory, and some in his hometown say that since he became speaker — and especially since President Trump took the White House — Mr. Ryan has lost his way. He was critical of Mr. Trump during the campaign. He largely holds his tongue now.
“I’m very disappointed in him,” Steve Johnson, 69, a former Janesville School District administrator, said while drinking coffee with a friend at Mocha Moment, a local cafe. Mr. Johnson, who says he votes Republican about half of the time, has known the speaker since Mr. Ryan was a student council representative, and has voted for him for years.
“He always had a good head on his shoulders, a good set of values,” Mr. Johnson said. “I supported him for all this time, but I won’t now, especially since he won’t stand up to Trump. That’s a big surprise to me.”
Others complain that Mr. Ryan no longer holds town-hall-style meetings (his aides say he hosts telephone call-ins where constituents can voice their concerns) and spends more time outside the district than in it.
“He’s just changed so much from the beginning of his career until now,” said Chris Rice, 68, a Democrat who works in health care. “I think he’s lost his values.”
But Mr. Ryan retains strong support elsewhere in the district, particularly in rural areas and counties like Waukesha, Walworth and Racine. His Republican backers insist that the speaker will be just fine, especially in the wake of the passage of the landmark tax overhaul, which he has been promoting during carefully controlled visits to businesses here and around the country.
“Ever since I’ve known Paul, tax reform was near and dear to his heart,” said Kim Travis, who represents Mr. Ryan’s district on the executive committee of the Wisconsin Republican Party. “So to get that passed is huge. I don’t think any of his opponents stand a chance.”
The most recent Marquette Law School poll found that, statewide, 46 percent of voters approve of Mr. Ryan, while 39 percent disapprove and 15 percent say they have not heard enough about him. And his aides note that the local economy has bounced back. The latest state figures show unemployment in Rock County, which includes Janesville, is at 3.6 percent. It was 13.2 percent in 2009, the year the auto plant closed.
Even so, some political observers are seeing cracks in the speaker’s armor. Stan Milam, a longtime Janesville journalist and former radio host, said that neither Mr. Bryce nor Ms. Myers has what it takes to beat Mr. Ryan this year. But he does see the speaker showing some signs of weakness.
“Every politician, if they have a lengthy political career, becomes more vulnerable than usual,” Mr. Milam said, “and I believe that’s where Paul Ryan is.”
Even Democrats concede that capitalizing on that will be an uphill battle. For one thing, they are fighting among themselves. In Washington, the Democratic Congressional Campaign Committee has endorsed Mr. Bryce. But many here see Ms. Myers, who now holds a seat on the Janesville School Board, picking up steam.
Her backers view Mr. Bryce as a creation of Hollywood and the national news media, and insist she is the more substantive candidate. They vow a strong grass-roots campaign.
“No one has been able to answer this for me: Why is Randy Bryce a better candidate to beat Paul Ryan?” Ms. Myers asked in an interview. “I’m talking about his qualifications, not his money.”
Mr. Bryce, 53, a longtime union activist who has waged three previous unsuccessful bids for local and state office, is pitching himself to voters as a man of the people, a kind of Joe Six-Pack who understands their everyday concerns. An Army veteran and a survivor of testicular cancer, he burst onto the political scene with a video campaign announcement that went viral.
In it, he offered to trade places with Mr. Ryan, declaring, “You can come work the iron, and I’ll go to D.C.”
The video captured the attention of national Democrats, and some in Hollywood. The liberal comedian Chelsea Handler hosted a $500-a-plate fund-raiser for Mr. Bryce, where she was pictured wearing a fake mustache.
But he has some baggage, which Ms. Myers and Republicans will undoubtedly try to exploit. He was late repaying a loan to an ex-girlfriend, and for nearly two years he owed $1,257 in back child support, which he paid after launching his campaign.
“I’m looking forward to that coming up; that’s why I’m running,” he said in an interview. “I’ve busted my rear end for the past 20 years to keep my head above water. That’s proof that I understand what it’s like to struggle.”
On the day the students marched last week through Janesville, demanding that Mr. Ryan take strong action on gun control, Mr. Bryce was among hundreds of people who turned up at a rally that was rife with anti-Ryan sentiment. Dressed in jeans, work boots and a paint-splattered, beaten-up brown work jacket, he hung back in the crowd, listening quietly as they shouted “Shame on Paul! Shame on Paul!”
When the speeches were over, he posed for selfies with the students and was mobbed by well-wishers. Among them was Teri Sickels, 59, a former assembly line worker who, like many here, was forced into retirement when the General Motors factory closed down.
“I love everything you stand for,” she gushed.
An earlier version of this article erroneously said Cathy Myers, a Democratic challenger to Paul D. Ryan, is a former schoolteacher. She continues to teach.
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