Over the first several months of his freshman term in Washington, Representative Rob Bresnahan Jr., Republican of Pennsylvania, became one of the most active stock traders in Congress, despite having campaigned for his seat on a promise to end stock trading by lawmakers on Capitol Hill.

Now Mr. Bresnahan’s trading is attracting more scrutiny after he made several transactions that appeared to benefit from the consequences of his own votes on the House floor.

Democrats are working to weaponize Mr. Bresnahan’s trades against him as they plot a campaign to oust him in next year’s midterm elections, and some Republicans privately concede that the issue could hurt him in his highly competitive district as they attempt to hold onto their slim House majority.

It is not just the striking number of trades that is dogging Mr. Bresnahan, a wealthy former chief executive of an electrical contracting company who was a top G.O.P. recruit. As of Thursday, he had made 626 stock trades since taking office, according to Capitol Trades, a site that monitors the stock market activity of lawmakers. That number made him the second-most-active trader this Congress.

It is also the possibility — or at least the appearance of one — that he could be benefiting financially from votes he made in Congress in favor of elements of President Trump’s domestic policy agenda that could end up harming his constituents.

On March 27, for instance, Mr. Bresnahan sold off between $100,001 and $250,000 worth of bonds issued by the Allegheny County Hospital Development Authority for the University of Pittsburgh Medical Center. The sale came after a report identified 10 rural hospitals in Pennsylvania that faced immediate risk of closure, and a month after Mr. Bresnahan voted for the House budget resolution that paved the way for large Medicaid cuts.

Mr. Bresnahan’s May 15 sale of up to $15,000 of stock in Centene, the largest Medicaid plan provider in the country, also drew scrutiny. Centene’s stock plummeted by about 40 percent after Mr. Trump signed the domestic policy bill into law on July 4.

Mr. Bresnahan has tried to blunt the political peril he is facing from his trading activity, but so far his efforts have fallen flat.

In May, he introduced his own bill to bar members of Congress from buying or selling stocks, though they would be permitted to keep holdings they had owned before the start of their terms.

If they wanted to keep trading, they would have to create a blind trust, in which an independent party manages investments with no input from the owner, to avoid conflicts of interest. The legislation would not take effect until 2027.

Mr. Bresnahan also announced that he was beginning the process of establishing a blind trust for his own assets.

“I want the people I represent to trust that I am in Congress to serve them, and them alone,” he said in a statement at the time.

But he has not followed through on that promise, complaining that the House Ethics Committee was making it difficult for him to wall off his stock holdings. At the same time, his trade volume continued to grow.

“The ethics process is prehistoric, and it’s downright excruciating,” he told WVIA News in a recent interview. “It’s been a long process that really leaves people that existed in the real world and came from different careers in a bind, when you want to provide a life of public service.”

Mr. Bresnahan balked at the idea of simply halting his stock trading. “And then do what with it?” he said in the interview. “Just leave it all in the accounts and just leave it there and lose money and go broke?”

Still, as the political scrutiny has grown, Mr. Bresnahan has appeared to at least slow his trading. It has been more than a month since he filed a periodic transaction report, which members of Congress who trade stocks are required to do regularly to report stocks they have bought and sold.

Mr. Bresnahan, who is getting married on Saturday, declined to be interviewed for this article. A spokeswoman, Hannah Pope, dismissed questions about his stock trading, which she called an “obsession,” and she would not say whether he had stopped, or intended to stop, trading stocks. She noted that Mr. Bresnahan’s stocks are traded by a financial adviser without his input, and that he learns of them when the public does.

More than 80 percent of voters support banning members of Congress from trading individual stocks, according to public polling. As outrage with the practice has grown, lawmakers in both parties have sought to appeal to growing populist sentiment by trying to outlaw the practice. Last month, a key Senate committee approved legislation that would bar members of Congress, the president and the vice president from trading stocks.

Some Republicans, however, have argued that such legislation would discourage wealthy people who have had successful private sector careers from entering public service.

In making his case against the Senate measure last month, Senator Ron Johnson, Republican of Wisconsin, said that banning congressional stock trading would “make it very unattractive for people to step up to the plate and run for office.”

Mr. Bresnahan, however, said that he wanted to end the practice.

“The public should never have to question whether their elected officials are serving the public or their own portfolios,” he said in a statement when he introduced his legislation.

But his legislation has not drawn a single co-sponsor, and he has not signed on to a stricter bipartisan bill that would impose an outright ban on stock trading by members of Congress, or require that they certify to congressional officials that they have established a blind trust for any holdings.

Mr. Bresnahan’s trading has made him a top target for the Democratic Congressional Campaign Committee and other outside groups seeking to reclaim the House majority.

“His prolific stock trading is more than just a broken promise,” said Eli Cousin, a spokesman for the D.C.C.C. “It’s political malpractice and a scandal of his own making that will define his time in Washington and be a major reason why he will lose his seat.”

Republicans, for their part, are counting on a crowded Democratic primary they say will push candidates to the left and make it impossible for any of them to win a swing district that voted for Mr. Trump. Despite the stock trading issue, they say they view Mr. Bresnahan, who has deep local ties to the area, as a good fit for the district, and a well-known name.

Maureen O’Toole, a spokeswoman for the House Republican campaign arm, said the focus on Mr. Bresnahan’s stock trading was part of “desperate Democrats’ false attacks to distract” from their agenda.

The Democratic mayor of Scranton, Paige Cognetti, is expected to challenge Mr. Bresnahan, according to a person familiar with her plans. Democrats are enthusiastic about her potential candidacy and expect Mr. Bresnahan’s stock trading to be a core part of the case they continue to make against him.

With so many trades to choose from, one Democrat involved in the race described finding fodder for attack ads as being “a kid in a candy shop,” and called Mr. Bresnahan’s stock portfolio as an “opposition researcher’s dream.”

To wit: the Natural Resources Defense Council, a nonprofit environmental advocacy group, recently began a six-figure ad campaign against Mr. Bresnahan in the district, focusing on the stock issue. It highlights how Mr. Bresnahan voted to repeal clean energy tax credits after having made between $200,000 and $1.4 million from stock trades that included selling clean energy stocks and buying up fossil fuel industry stocks.

“He made out like a bandit,” the ad says. “Tell Rob to stop trading stocks in Washington.”