U.S. Attorney for the District of Columbia Ronald C. Machen Jr. (Jacquelyn Martin/AP)

The exploits of U.S. Attorney Ronald C. Machen Jr. make for a compelling crime drama, a serial, even.

Endowed with enormous prosecutorial powers, he leads a stealthy team of legal eagles through a shadow world of shady people, greasy palms and sticky fingers. And they really stick it to the wrongdoers. In an ongoing, three-year probe of public corruption in the District, Machen and his white-collar crime-busters have squeezed guilty pleas out of everyone who has been charged so far.

Like all crime-fighting superheroes, however, our Machen appears to have a potentially fatal flaw. Inside the prosecutor dwells a preacher whose penchant for proselytizing sometimes casts doubt on the lawman’s impartiality.

For instance, it is a cardinal sin for a U.S. attorney to give the appearance of influencing an election. Especially in the aftermath of what some people saw as the politically motivated investigations that occurred under Attorney General Alberto R. Gonzales during George W. Bush’s presidency.

And yet, as voters head to the polls Tuesday for the Democratic mayoral and D.C. Council primaries, Machen’s fingerprints will be on every ballot box.

In exchange for a slap on the wrist for making illegal campaign contributions, businessman Jeffrey E. Thompson told Machen that Mayor Vincent C. Gray (D), who is seeking reelection, knowingly accepted illegal money for his 2010 election bid. Instead of proving the allegation in court, Machen put the hearsay in the streets at a March 10 news conference.

His words were still reverberating through the campaign a week later on March 17, when early voting began in the primaries.

“Jeff Thompson’s guilty plea pulls back the curtain to expose widespread corruption,” Machen said at a news conference earlier this month. He did not mention Gray by name. What he did say was that the person who took the illegal money was the same person who won the mayoral race in 2010.

Of course, that would be Gray.

“[The Thompson] plea gives the citizens of D.C. an inside look at the underground, off-the-books schemes that have corrupted election after election, year after year,” Machen said.

So saith the preacher. But the prosecutor has yet to make his case.

Until that news conference, Gray had been leading the mayoral race by as many as eight points. Not long after Machen became part of the campaign, however, polls showed the race to be a dead heat between Gray and Council member Muriel Bowser (D-Ward 4).

Bowser is a protege of former mayor Adrian M. Fenty (D), who lost his reelection bid to Gray in 2010. To some Gray supporters, it appeared as if Machen was helping the die-hard Fenty faithful get payback by proxy.

“Now who’s running a ‘shadow campaign’?” said Rahim Jenkins, a community activist in Southeast Washington. “Is this a case of the pot calling the kettle black?”

In the Bush years, some prosecutors were found to have directed investigations specifically aimed at crippling liberal candidates. Some were said to have ordered probes into “voter fraud” with the intent of suppressing the black vote.

The stench of prosecutorial misconduct certainly has not dissipated. Despite Machen’s admirable work with ex-offenders in the District and his commitment to building trust in black communities, there is still too much bad blood between many African Americans and the “justice system” for him to take lightly the appearance of fairness.

In the opening episodes of this unfolding drama — call it the Machen Squad vs. D.C.’s Dumbest — the prosecutor took down a former D.C. Council member and two other council members who were still in office. As head of the nation’s largest U.S. attorney’s office, he had offered just a glimpse of the awesome resources and broad discretion he can use in tracking down fraud, theft and bribery, among other kinds of public corruption.

Nevertheless, Machen implies that he was powerless not to accept Thompson’s plea at the time. Why he had to sermonize about it at a news conference remains a mystery.

“Jeffrey Thompson was finally ready to enter a guilty plea and acknowledge his crimes when he signed a plea agreement on March 7,” according to a statement released by the U.S. Attorney’s Office. “Mr. Thompson’s guilty plea was scheduled for the next available court date rather than concealed from the public until after the election.”

It almost sounds as if Thompson was calling the shots — timing his plea to get the U.S. attorney to drop a slandering bomb on a mayoral candidate right before an election.

Suspense mounts in anticipation of Tuesday’s vote results. Will Machen’s late play contribute to an upset? Will Gray loyalists be the ones looking for payback next time?

Stay tuned.

In future episodes, perhaps the prosecutor will let a conviction speak for itself and lock up that loose-lipped preacher until the season finale.

To read previous columns, go to washingtonpost.com/milloy.