Perhaps no federal officeholder in modern American history has been accused of ignoring, testing or breaking as many aspects of campaign finance law so flagrantly, in such a short span of time, as George Santos has.

But his case, while sensational, illustrates the profound weaknesses of the system, and its potential for abuse.

For years, campaign finance laws have eroded, while the watchdogs responsible for their oversight have been weakened by limited powers, underfunding and political stalemate. The system, which largely relies on campaigns and political committees to self-report thousands of donations, expenditures, loans and refunds, has been left wide open for anyone willing to mislead, experts said.

Mr. Santos might have slipped through unnoticed — and many candidates probably do.

“He is an extreme example of something that is happening all the time in campaign finance,” said Saurav Ghosh, a former Federal Election Commission enforcement lawyer who is now the director of federal campaign finance reform at the Campaign Legal Center, a watchdog group. Mr. Santos, he suggested, was able to take advantage of “the overall under-regulation of money that is raised and spent on election influence.”

While a handful of congressional candidates have been convicted of misusing campaign money in recent years, Mr. Santos — who faces federal charges but has not been convicted of any crimes — had the acute misfortune of actually winning his race, and of representing a district saturated with attention from the news media and federal law enforcement.

“The regulation of campaigns and how they spend their money is inadequate to prevent this from happening,” said Brett Kappel, a campaign finance lawyer. “You can just run for office, spend the money on yourself, and if you lose,” possibly escape unnoticed. “He got in trouble because he won.”

He added that Mr. Santos could paint the way for future candidates to abuse the campaign finance system: “The thing is, people see this, and they think: This is a great scam.”

Mr. Santos, a Republican who was elected in 2022 to represent a district on Long Island, was expelled from Congress on Friday after a report by House investigators found “substantial evidence” that he had committed federal crimes. The report detailed how Mr. Santos had diverted tens of thousands of dollars of campaign money toward personal expenses, including luxury goods, Atlantic City casinos, rent payments and a website primarily known for explicit sexual content.

The report came nearly a year after The New York Times detailed a pattern of embellishment and major inconsistencies in Mr. Santos’s résumé, setting off state and federal investigations and a House ethics inquiry.

Mr. Santos also faces federal charges that he and his former campaign treasurer conspired to file false reports with the F.E.C., among other campaign finance-related charges. He also faces criminal counts including unemployment insurance fraud and making false statements. He has pleaded not guilty to all of the charges, and is expected to go to trial in federal court on Long Island. (The former campaign treasurer and a fund-raiser have pleaded guilty to federal crimes in connection to their work with Mr. Santos.)

Carrie H. Cohen, a former federal prosecutor in New York who handled a number of high-profile public-corruption cases, said the extent of Mr. Santos’s reported campaign finance misconduct might never have been discovered except for the fact that people started paying attention to his back story.

“All this came to light because there were so many other complaints against him,” she said. Otherwise, she said, it can be hard for the F.E.C. or federal prosecutors to spot wrongdoing in campaign finance reports.

Over the years, plenty of members of Congress have been charged with federal crimes like tax evasion, accepting bribes, insider trading and obstruction of justice. And in the decades since the modern campaign finance system came into being, in the wake of Watergate, some have been wrapped up in campaign finance scandals.

In 2012, Jesse L. Jackson Jr., a Democratic congressman from Chicago, resigned from Congress amid a federal investigation into misuse of campaign money. Months later, he pleaded guilty to a felony fraud count related to spending $750,000 of campaign funds for personal use.

In 2019, Duncan Hunter, a Republican congressman from California, pleaded guilty to charges that he spent more than $200,000 in campaign money on personal use, after federal prosecutors charged him in a sweeping corruption case. He stepped down in January 2020 and was later pardoned by former President Donald J. Trump.

But Mr. Santos is unique in the scale of his reported misconduct. The allegations against him are not simply that he used his campaign as a “personal piggy bank” — to use a term favored by federal prosecutors — but that nearly every aspect of his campaign contained elements of fraud.

A major criticism of the F.E.C. in recent years is that its bipartisan structure — three Republican commissioners and three Democratic commissioners — has resulted in deadlock and inaction, meaning reports of potential violations go unpunished. The commission has also struggled to recruit and retain staff members, even as the amount of political transactions across the system has drastically increased.

A spokeswoman for the F.E.C., which oversees the campaign finance system, declined to comment. The commission’s leadership has previously pushed back on assertions that it is falling short.

In an April statement to The New York Times regarding a different matter, the F.E.C.’s chairwoman, Dara Lindenbaum, a Democrat, and its vice chairman, Sean J. Cooksey, a Republican, said: “We take this agency’s enforcement and transparency mission seriously, as do all of our colleagues. Without commenting on any specific case, Commissioners assess each enforcement matter on its merits, and we reach agreement in nearly 90 percent of them. Any claim that the Commission is ‘toothless’ or that its bipartisan structure prevents it from fulfilling its mission is misinformed.”

Even as the F.E.C. sought to respond to Mr. Santos’s actions with its usual enforcement mechanisms, those tools were not on their own able to address the extent of the brazen misconduct he has been accused of. In the months before Mr. Santos’s election, the F.E.C. sent his campaign a series of “requests for additional information” — commonplace inquiries seeking details about transactions not fully accounted for in a committee’s regular filings with the commission.

The F.E.C. does not have the power to look in bank accounts and must take campaign finance disclosure reports at face value. Unlike the Securities and Exchange Commission, it is not a regulatory agency — its primary mission, laid out in its most recent annual report, is to provide transparency and “promote compliance.”

Some questions about Mr. Santos’s campaign remain unanswered. His campaign’s filings are filled with suspicious expenditures — for example, some of the dozens of charges pegged at $199.99, cents below the federal threshold that requires receipts — and outright omissions, such as hundreds of thousands of dollars in spending that was never accounted for.

“Unfortunately, we don’t know what we don’t know,” said Paul S. Ryan, a campaign finance expert who previously worked at the Campaign Legal Center. “The bottom line here is that the only thing the F.E.C. sees, in most instances, at first glance, are the campaign finance disclosure reports,” he said. “And if someone is smart enough to simply lie about what they are using it for,” then it can go overlooked, he said.

Personal enrichment is explicitly disallowed by campaign finance law. But campaigns commonly push the envelope on disclosure rules that are aimed at providing transparency about the source of campaign contributions and how that money is spent.

The presidential campaign of Senator Tim Scott of South Carolina routed a majority of its reported $20.7 million in spending through two limited liability corporations whose addresses were suburban Staples stores, untraceable to any single vendor. Mr. Trump’s 2020 campaign was the subject of litigation over its use of limited liability companies that received hundreds of millions of dollars and were run by campaign staff and family members.

“Federal campaign finance law applies more regulations to money going into campaigns than going out of campaigns,” Mr. Ryan said. “Almost all of the attention of federal law is on the fund-raising side, not the spending side.”