Dean’s Car Care, an auto repair shop in Portland, Ore., used to regularly rack up five stars and gushing accolades on Yelp and Google Reviews for its reliable and friendly service.

“Honest and affordable. What else could you ask for?” one happy repeat customer wrote online in 2016. The reviewer said he had moved to a different part of the city, “but I still drive up to Dean’s whenever I need work done on my car or my truck.”

That was then, before the owner, Marie Gluesenkamp Pérez, was elected to Congress last year as a Democrat, and became one of only a small number of lawmakers in her party who periodically crosses over to vote with Republicans.

These days, Ms. Gluesenkamp Pérez is one of the most vulnerable Democrats in Congress, and Dean’s — the family business named for her husband — has become the target of vicious online trolling from the left. Progressives from around the country are review-bombing the establishment with posts expressing their ire at the first-term congresswoman for siding with Republicans on a bill to repeal President Biden’s student loan relief initiative.

“Worst car care Ive been to,” one Yelp reviewer wrote in a now-typical post in May, shortly after Ms. Gluesenkamp Pérez cast that vote. “My car was dirtier than when I dropped it off.” He added: “It also didnt help that Marie was bragging about her PPP loan and in the same sentence said f students. Shes very hypocritical.”

Added another: “This place is horrible. They charge interest that compounds daily. Ohh wait that’s student loans. This place really is the worst.”

The online abuse may be an occupational hazard of serving in Congress at a tribal moment in politics, when there is little room for lawmakers to break with their own party, no matter the circumstances or the issue.

Ms. Gluesenkamp Pérez, 35, flipped her Washington State district blue in November in one of the country’s most competitive House races, after a campaign that highlighted her biography as a middle-class small-business woman and often featured her auto shop in advertisements and photos. In a Democratic victory few saw coming, she beat a Trump-backed former combat veteran, Joe Kent, who denied the legitimacy of the 2020 presidential election and supported defendants charged in connection with the Jan. 6, 2021, attack on the Capitol.

Ms. Gluesenkamp Pérez represents a largely rural, middle-class district, where only a quarter of residents are college graduates. Donald J. Trump carried her district in two consecutive presidential elections. She has criticized her party for being out of touch with middle-class voters and railed against Democratic elitism.

“The whole premise of higher education is that you earn more money,” she said in an interview. “It’s ironic to protest a mechanic shop when you’re sitting on your freshly minted graduate degree. I’m here to represent my district, not inner-city New York.”

Back in May, when she cast the fateful vote, Ms. Gluesenkamp Perez explained her rationale. “Expansions of student debt forgiveness need to be matched dollar for dollar with investments in career and technical education,” she said at the time. “I can’t support the first without the other. The severe shortage of trades workers needs to be seen and treated as a national priority. It’s about respect.”

None of her explanations have quelled the trolling, which is part of a new online frontier in political discourse, in which voters from across the country can express their anger at elected officials who may not even represent them by anonymously dragging down small businesses associated with one of them. Some members of Congress who own small businesses said they have tried hard not to have their names associated with their businesses at all for fear of this type of retribution.

“Awful company with terrible owners,” a reviewer said in a May post about Ms. Gluesenkamp Pérez’s shop. “Pay back your PPP loan you frauds.”

One user dubbed her “crooked gluesenkamp,” echoing Mr. Trump’s favorite moniker for Hillary Clinton, and suggested she owed customers free carwashes because of her voting record.

The rush of negative reviews against Ms. Gluesenkamp Pérez’s business appear to have been started by users who were livid with her decision to break with Democrats in May and vote to repeal Mr. Biden’s student debt relief program. She was one of just two House Democrats to do so, along with Jared Golden of Maine, who is also from a competitive district.

The reviewers point to what they view as hypocrisy by Ms. Gluesenkamp Pérez, whose small business received a $63,000 loan from the federal Paycheck Protection Program, created during the pandemic to keep such outfits afloat.

“Horrible service,” another reviewer noted. “Ran by a woman who also took a PPP Loan for her business and then decided to vote against student loan forgiveness.”

Ms. Gluesenkamp Pérez said the online insults were inappropriate but would not succeed in dragging down her business or changing how she does her job in Washington.

“The Google reviews aren’t going to change the way that I vote,” she said in an interview. “It’s unfortunate because student loans is a very nuanced policy issue. When you boil it down to this hate, this internet-based trolling, it’s not productive. I’m here to listen to my constituents. I hold regular in-person town halls.”

The shop, she added, has “plenty of work to do, a strong reputation, and our community knows what we do.”

Ms. Gluesenkamp Pérez — who serves as a co-chairwoman of the Blue Dog Coalition, a group of centrist Democrats who are outspoken on national defense and fiscal issues — is a top target of Republicans. Her district is rated by the nonpartisan Cook Political Report as a tossup.

She is also young, with blunt-cut bangs, and often wears jeans or wide-leg pantsuits, looking more as if she might be aligned with the progressive Squad than the centrist that she is. People close to Ms. Gluesenkamp Pérez said that incorrect assumptions about her politics, based on her appearance, only intensify the anger by some on the left about her voting record.

Ms. Gluesenkamp Pérez was one of just four Democrats to vote this month for the annual defense bill, which Democrats broadly rejected after Republicans added provisions that would limit abortion access, transgender care and diversity training for military personnel.

She was one of just seven Democrats who voted with Republicans to condemn the use of elementary school facilities to provide shelter for undocumented immigrants who are not admitted to the United States. And she voted “present,” rather than “yes,” on a measure to expel George Santos, the embattled Republican representative from New York, from Congress. Unlike most Democrats, she opposes a ban on assault weapons, though she has expressed support for raising the age for purchasing one from 18 to 21.

Her Democratic colleagues in Congress are more understanding of the dynamics of the district she represents, and the very real threat she faces in the 2024 election.

“She’s very endangered,” said Representative Donald S. Beyer Jr., Democrat of Virginia, who voted against the defense policy bill but said Ms. Gluesenkamp Pérez deserves “a pass” on such issues.

After the defense bill passed, Mr. Beyer said he received emails from Democrats around the country saying, “‘You’ve got to get rid of Marie Pérez.’ I wrote back. I said, ‘No, she’s fine. We love her. Do you want Kevin McCarthy to be speaker forever?’”