WASHINGTON — As House ethics investigators were examining four cases this fall detailing a sweeping array of improper financial conduct by lawmakers, they ran into an obstacle: Two of the lawmakers under scrutiny refused to meet with them or provide documents.

The investigators were not too surprised. Over the past decade, fewer and fewer House members have been willing to cooperate with congressional investigations, a development that ethics experts warn could reduce accountability for misdeeds and erode trust in the institution of Congress.

Omar Ashmawy, the staff director of the Office of Congressional Ethics, an independent entity that reviews allegations against House members and refers misconduct cases to the House Ethics Committee, acknowledged the growing resistance to his office’s work, but said he was undeterred: “It has never prevented us from being able to gather the facts and determine what happened and whether or not the subject was culpable.”

Still, the trend is unmistakable.

In 2009 and 2010 — the first Congress scrutinized by the Office of Congressional Ethics, which was created in 2008 — three lawmakers refused to cooperate with the office’s 68 investigations, a noncooperation rate of just 4 percent.

This year, six out of 14 House lawmakers under investigation have refused to participate — a rate of 43 percent, the highest on record.

Mr. Ashmawy said he believed the higher noncooperation rate was a result of investigators focusing on fewer cases, and more potentially serious ones. Lawmakers who may be glad to meet with ethics investigators to clear up an honest mistake or minor oversight are more reluctant to cooperate if they think they may face weightier consequences, he said.

There is no requirement that lawmakers cooperate with the Office of Congressional Ethics, but legislators who do so often are able to resolve what had appeared to be violations of ethics rules.

The fact that many will no longer even meet with ethics investigators reflects a troubling trend in American politics in which improper behavior is no longer a political liability, ethics experts say.

“There’s a trend towards not taking ethics rules seriously and also more resistance to cooperating in ethics investigations or, frankly, even acknowledging the legitimacy or authority of ethics investigation,” said Bryson B. Morgan, a lawyer at the firm Caplin & Drysdale in Washington, who was previously an investigative counsel for the Office of Congressional Ethics. “I think there’s been a bit of a backsliding on ethics.”

Mr. Morgan said he believed ethical norms changed during the Trump era. As allegations piled up against President Donald J. Trump with no apparent consequences, many politicians concluded that ethical behavior no longer mattered and decided to stop cooperating with congressional investigations, he said.

“What people used to think was a career-ending mistake has been proven to not be a career-ending mistake,” Mr. Morgan said. “Many people have noticed a shift in ethical norms. It used to be the case that when a member violated the ethics rules, if not a fine, there would be a fairly stiff political price to pay. I worry that has gone away.”

The two lawmakers who refused to cooperate in cases examined this fall were Representative Mike Kelly, Republican of Pennsylvania, who is under scrutiny over stock purchases by his wife that investigators say were influenced by his actions as a member of Congress; and Representative Jim Hagedorn, Republican of Minnesota, whose office is accused of improperly awarding contracts to companies owned by his aides’ relatives.

Those cases now rest with the House Ethics Committee.

A spokesman for Mr. Kelly did not respond to a request for comment.

But Elliot S. Berke, an attorney for Mr. Hagedorn, said he was working directly with the Ethics Committee, rather than the Office of Congressional Ethics, to try to resolve the matter.

“Congressman Hagedorn will continue to work with the Ethics Committee to bring this matter to a conclusion,” Mr. Berke said. “Moreover, he had no knowledge of the underlying issues and has acted in good faith throughout.”

Congress rarely takes serious disciplinary steps against its own members, unless their ethical misdeeds rise to a federal crime. The last House member expelled was James A. Traficant Jr., Democrat of Ohio, in 2002, after he was convicted of 10 felony counts including bribery, racketeering and tax evasion.

In 2020, the Senate Ethics Committee received 144 complaints of violations and dismissed them all.

Kedric Payne, the senior director for ethics at the Campaign Legal Center and a former deputy chief counsel for the Office of Congressional Ethics, said it was becoming all too common for lawmakers to flout the ethical rules Congress has imposed. The Campaign Legal Center has tracked dozens of violations of the STOCK Act — which requires members to report stock trades within 45 days of the transaction, but is commonly ignored.

Business Insider reported this month that 52 members of Congress violated the act this year. Penalties are often minimal, beginning at just a $200 fine, and lawmakers are allowed a 30-day grace period to comply after missing the deadline.

Speaker Nancy Pelosi, Democrat of California, recently dismissed the idea that members should be prohibited from trading stocks.

“We’re a free-market economy,” she told reporters. “They should be able to participate in that.”

But Mr. Payne argued more needed to be done to ensure members of Congress were behaving ethically. Refusing to comply with the STOCK Act or to cooperate with ethics investigations should not be accepted as routine, he said.

“Noncompliance with the STOCK Act is the most blatant violation by multiple members of Congress that I’ve seen in recent history,” he said. “You need stronger rules that would restrict stock trades that appear to be conflicts of interest — for example, trading stock in an industry that is within the jurisdiction of your committee.”

Still, he believes participating with a congressional investigation is likely the smarter strategy for a lawmaker.

In 2019, Representative David Schweikert, Republican of Arizona, refused to cooperate with an Office of Congressional Ethics investigation into campaign finance violations and allegations that he misused funds and pressured his congressional staff to perform campaign work. He later become the first member reprimanded on the House floor since 2012.

“If you don’t cooperate, there’s a higher likelihood that the O.C.E. will find substantial reason to believe that a violation occurred,” Mr. Payne said.