Since the day she began her campaign for Congress, Paige Cognetti, the Democratic mayor of Scranton, has centered her pitch to voters on the prodigious stock trading of the House Republican she is seeking to unseat, Representative Rob Bresnahan Jr. of Pennsylvania.

“This year he’s already made 600 trades,” Ms. Cognetti said in the announcement video for her campaign in September. “Sold Chinese stocks the week of a tariff. Buys missile stocks before a war. Buys jet stocks before a big contract. Oversees crypto? Buys crypto stocks before a vote.”

It may be the district where the issue rings the loudest. Democrats are running ads highlighting how Mr. Bresnahan profited from a data center development by trading stocks related to the industry, and recent polls show Ms. Cognetti with a narrow lead. But she is not alone in zeroing in on stock trading by members of Congress as a potent line of attack in this year’s midterm elections.

Like many members of Congress who trade substantial amounts of stock, Mr. Bresnahan, the wealthy former chief executive of an electrical contracting company, has said he relies on a financial adviser to make his trades and never knows about the transactions before they happen or when they occur.

Mr. Bresnahan’s campaign said that Ms. Cognetti’s descriptions of his trades were “lies” and that he has “taken legitimate steps to ban congressional stock trading.”

Still, at a time when many voters are experiencing economic hardship and have grown increasingly suspicious that the political system is rigged, the mere possibility that a member of Congress could be profiting from the job has become politically toxic.

Roughly a quarter of legislators on Capitol Hill have traded individual stocks this term, with some buying and selling millions of dollars in shares. Since the start of this term in January 2025, members and their families have collectively traded $160 million to $664 million in individual stocks, according to a New York Times analysis of data collected from members’ required financial filings by 2iQ Research, an analytics firm.

An overwhelming majority of voters in both parties view stock trading by lawmakers with great suspicion and say they support barring them from doing so. The public backlash has fueled efforts by members of both parties, so far unsuccessful, to enact new legislation banning the practice.

In the meantime, both sides are leveraging the issue in the fight for control of Congress.

Democrats and Republicans alike are seeking to weaponize incumbents’ trades, particularly those where they can show the appearance of a conflict of interest, and putting them at the forefront of ads and campaign attacks in the country’s most competitive districts. It is a way to portray their opponents as corrupt and out of touch with the financial realities of working-class Americans.

In Northern New Jersey’s competitive Seventh Congressional District, the Democratic Congressional Campaign Committee has been targeting Representative Thomas H. Kean Jr. for continuing to trade stocks while missing more than 100 votes during an extended absence for what he later said was treatment for depression. Mr. Kean’s office did not respond to a request for comment.

In Virginia, Democrats are attacking Representative Rob Wittman for trading more than $2.5 million in stocks between 2015 and 2023 while serving in Congress — including buying defense stocks like Lockheed Martin and Honeywell while serving on the Armed Services Committee.

“There’s a vicious cycle in Washington: Politicians trade stocks in the industries they’re supposed to regulate, then protect those industries instead of you,” said the caption with a campaign video that his opponent, Shannon Taylor, a prosecutor, posted online. “Let’s ban congressional stock trading — for good.”

(An outside group supporting Mr. Wittman apparently views this line of attack as effective: This week, it began running an ad highlighting his vote for a watered-down stock trading bill that passed the House passed in July, noting that he voted to put an end to the “greed and corruption” of “fat cat lawmakers.”) Mr. Wittman’s office did not respond to a request for comment.

Republicans have also identified the issue as resonant with their voters.

In New York’s competitive Third Congressional District on Long Island, Michael J. LiPetri Jr., a Republican former state assemblyman, routinely attacks the Democrat he is running to unseat, Representative Tom Suozzi, for “the millions he’s pocketed from stock trading in Congress.”

Mr. Suozzi made nearly 600 stock trades involving 161 companies totaling as much as $24 million when he served in Congress between 2017 and 2023, according to public filings. In 2021, he faced an ethics investigation for failing to report about 300 stock trades in the required 45-day window. The House Ethics Committee later cleared him after concluding that it was not a willful violation, but a mistake.

A spokeswoman for Mr. Suozzi said all of his purchases were made at the discretion of his broker, without his direct input. She added that he has stopped purchasing stock altogether since returning to Congress in 2024, although he has sold some.

And in Nevada, Marty O’Donnell, a Trump-endorsed Republican running against Representative Susie Lee, a Democrat, often highlights Ms. Lee’s stock trades, claiming that she “has used her office to enrich herself with information the rest of us don’t have.” He has pledged that “unlike Susie, I will not trade stocks while representing Nevada in Congress.”

Ms. Lee, who was married to the chief executive of Full House Resorts, which operates casino facilities, has traded up to $2.8 million worth of stock since her divorce in 2024.

A spokeswoman for Ms. Lee said that all trades done before her divorce were executed by a money manager or her ex-husband. Since the divorce, Ms. Lee “received stocks as part of her divorce settlement and committed to divesting from them, which involves the selling of individual stocks in order to invest in other allowable instruments, like E.T.F.s and mutual funds,” the spokeswoman said, referring to exchange-traded funds.

In one instance where a former lawmaker is running for her old seat, an incumbent is trying to use the attack that is typically employed by a challenger.

Representative Jen Kiggans, Republican of Virginia, frequently accuses her opponent, former Representative Elaine Luria, of “insider stock trading” during her four years in Congress. Ms. Kiggans’s campaign did not point to any instances of insider trading, which is illegal, when asked about the allegations.

Instead, the campaign highlighted Ms. Luria’s previous opposition to a congressional stock trading ban (she now supports one). And it noted that Ms. Luria owns between $5 million and $25 million in NVIDIA stock, which she held when she voted for a $280 billion industrial policy bill enacted in 2022 that provided tens of billions of dollars for semiconductor manufacturing and research.

A spokesman for Ms. Luria dismissed Ms. Kiggans’s attacks as “lies,” and said that the bill in question did not increase her stock value. “Elaine supports banning congressional stock trading and preventing politicians from using their public offices for private gain,” he said.

The campaign trail focus comes as House Republicans have recently pushed through a bill that would impose some restrictions on lawmakers who trade stock, but stops short of a more stringent ban backed by a bipartisan coalition.

Representative Hakeem Jeffries, Democrat of New York and the minority leader, has promised to bring the bipartisan alternative to the floor if Democrats win control of the House next year.

Donald K. Sherman, the chief executive of Citizens for Responsibility and Ethics in Washington, a Democratic-aligned watchdog group, said it was not surprising to see the attacks on the practice coming from both political parties, because the issue is more salient than some others in government.

“You don’t need to be an ethics expert to understand it,” Mr. Sherman said. “An individual member of Congress can have an impact on a stock price, whether it’s tough questions at a hearing or an oversight hearing or confirmation of a particular cabinet secretary.”

Former Representative Tom Malinowski, Democrat of New Jersey, whose political career ended in part because he did not properly disclose dozens of stock trades, said a stock trading ban would do little to address far more serious issues that have eroded the public’s trust in political leaders.

“Stock trading by members of Congress should be banned, but it’s still a very poor substitute for attacking the real sources of corruption in Washington: unlimited dark money and Trump’s blatant selling of the presidency,” Mr. Malinowski said in an interview. “Those are significantly bigger problems. But candidates have seized on it as an all-purpose explanation for corruption.”

Olivia Diaz and Andrea Fuller contributed reporting.