Tim Day remembers clearly when he found the “smoking gun” that now seems certain to send former D.C. Council member Harry Thomas Jr. (D) to federal prison.

It was in September 2010. Day, who was Thomas’s Republican opponent for the Ward 5 council seat, was in bed.

“It was about 1 o’clock in the morning, and I was there with my laptop,” Day recalled. He was poring over apparent irregularities in records of Thomas’s purported charity work for youth sports programs.

“One plus one didn’t equal two, and it was driving me crazy,” he said. “The ‘aha’ moment was when I realized that he had not bothered to attain [a] nonprofit status for his entity, Team Thomas.”

That potentially made donations to the group illegal. It also helped explain what was really happening to about $300,000 contributed to the program.

“I realized, ‘What’s he doing? He’s taking the money!’ ” Day said.

It’s a nice story, with a satisfying ending for anybody who cares about honesty in government and holding politicians accountable.

On Friday, Thomas pleaded guilty to two felonies connected to his theft of more than $350,000 of public funds for personal use, including golf vacations and a sport-utility vehicle. After more than a year of categorically denying that he’d done anything wrong, Thomas resigned and admitted that he broke the law for years in pretty much exactly the way Day figured out 16 months ago.

But the tale also offers two separate lessons, which are much less satisfying. It demonstrates the weakness of the D.C. government’s internal accounting systems. And it highlights anew the tendency in much of the city’s Democratic political establishment to ignore — or at least minimize — evidence of political corruption.

Day points out that a number of government watchdogs had missed chances to catch Thomas’s crooked dealings.

“No one found that this money was misappropriated, but little old me was able to sit in front of my computer and find and track it all,” Day said.

“The council, the Children [& Youth Investment] Trust, the auditor and [Chief Financial Officer Natwar M.] Gandhi’s office all failed to do their job. That is a systemic failure,” he said.

In the end, Thomas was convicted because D.C. Attorney General Irvin B. Nathan and the U.S. attorney’s office were persuaded by Day’s work.

Former U.S. prosecutor Joseph E. diGenova said the case was “handed to the U.S. attorney on a silver platter.”

If the legal authorities were impressed, however, the politicians stayed mostly mum. There was little outrage at the top when Day first went public with his findings, even though he outlined them in detail in a 27-slide PowerPoint presentation.

“It’s a well-known fact that in D.C. we allow suspicious behavior, and we don’t come forward,” Day said. “We have to break the mentality in this city that it’s okay for bad things to happen.”

From the beginning, Democratic bosses in Ward 5 and elsewhere dismissed the allegations as a Republican smear campaign. When this newspaper’s editorial page pounded Thomas for failing to explain the anomalies, the criticism was written off as a news media vendetta.

Even when Thomas agreed to repay $300,000 to the District to settle a civil suit by Nathan’s office and the case was referred to the U.S. attorney’s office for criminal investigation, only three of the 12 other D.C. Council members called for Thomas’s resignation.

Day said he believed that ordinary residents are scared to criticize their representatives for fear of retribution and that politicians protect one another at the public’s expense.

“There’s the good-old-boy network in the District of Columbia: ‘I do this for you, you do this for me,’ ” Day said. He knows from personal experience — in e-mails, text messages and phone calls from random critics — of the toll paid by anyone who speaks up.

“Every day, I have to hear how I should be ashamed of myself for ruining [Thomas’s] life, and ruining his family’s life,” Day said.

When he’s not battling public corruption, Day, 40, works as a tax accountant. He has lived his whole life in the District and graduated from the city’s public schools. He went into politics because he was unhappy about city taxes and poor police performance in his neighborhood.

Some local political leaders think his work will make a difference.

“People, at least for a time, are going to be looking much more closely at money issues,” said John Salatti, a Democratic Advisory Neighborhood Commission member in the ward. “When Tim Day comes out with something else on somebody, they're going to give it a little more credence, and not just fob it off as Republican dirty tricks.”

That’s a lesson that good government supporters from either party could support.