Trump is relentlessly arguing that Clinton is unfit for office

Think of it as the moment when Donald Trump truly learned to throw a (campaign) punch. It came about three weeks ago, amid the latest swirl of stories on the  Clintons’ ethics.  Hillary Clinton had recently blamed her private email server on  Colin Powell. Judicial Watch had released more emails that showed the Clinton Foundation begging the State Department for special favors on behalf of its donors.  Bill Clinton had floated laughable plans to reform the foundation.

The Trump campaign pounced. It began blasting out every new revelation about—or editorial-board comment on—Mrs. Clinton’s shady dealings. It unleashed surrogates, in particular the former prosecutors  Rudy Giuliani and  Chris Christie, to make the legal case against her.  Mr. Trump devoted a significant portion of a speech in Texas to detailing the lies she had told about her server, and the pay-to-play allegations at the foundation. Within a few days the campaign had cut a web ad hitting her for “corruption” and calling the foundation a “slush fund.”

Mrs. Clinton’s slip in the polls is a direct result of the latest flood of scandal. Less noticed is the skillful way that Team Trump is making those hits land. The Republican nominee’s campaign has been doing more right lately, though nothing more so than this. When it comes to the dissection of Mrs. Clinton’s misdeeds, the Trump campaign is firing on 16 cylinders.

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If attacking your corrupt opponent on corruption seems obvious, it wasn’t to Mr. Trump for a long time. The GOP nominee is a scrapper, and part of his draw was the expectation that he would speak bluntly about the Clintons. He did, though as voters would soon realize, only in fits and incomplete starts. He delivered a speech on her ethics in June—then never sustained the argument. He ignored prime opportunities (the State Department inspector general report; FBI Director  James Comey’s press conference), flitting to other subjects instead. He seemed to think the occasional #CrookedHillary tweet was enough.

This was frustrating if only because Mrs. Clinton’s venality is the concrete with which Mr. Trump must pave his road to the White House. This is a woman whom close to two-thirds of voters view as untrustworthy. She has based her entire campaign around the argument that she is more credible and competent to lead the nation—a claim utterly undermined by revelations about her foundation’s business model, her cavalier handling of classified information, and her inability to “recall” most of her tenure as secretary of state.

Mr. Trump’s new approach is to unrelentingly hit all sides of that claim—making the case that Mrs. Clinton is “unfit” to hold the top job. Case in point: After Wednesday’s commander-in-chief forum on NBC, the Trump campaign detailed precisely why Mrs. Clinton should not be trusted with national security. Mr. Trump hit her during the forum, while a follow-up press release highlighted her bad judgment in using an “illicit” email server and the risk of a “hack of classified info”; noted how her drone emails would “undermine” security; and flagged her stumbling attempt to tell a vet in the audience why she should be held to a different standard on classified information than military officers. Good, smart stuff.

The unrelenting pressure has put Mrs. Clinton in the spotlight and on the defense. That by consequence has somewhat sheltered the Republican nominee from attacks. Listening to Mrs. Clinton—the subject of an FBI investigation, the facilitator of the Crown Prince of Bahrain—on Wednesday drop a line about Mr. Trump’s “scams” and “frauds” was downright amusing. Sort of like watching  Al Capone accuse  Eliot Ness of having the occasional light beer.

Also effective has been the campaign’s drumbeat about Mrs. Clinton’s refusal to hold press conferences. (“Hiding Hillary: Day 278!”) Her longtime approach to scandals has been to ignore them and wait for the press to get weary. But the stories keep pouring out this time, and her refusal to address them has made her look shady, arrogant, slippery. She finally succumbed to pressure and took a few cursory questions from reporters Thursday, though none on the email or foundation.

Mr. Trump’s new all-in-on-Hillary approach came about the same time as his staff shakeup. It is likely due in part to the presence of Breitbart’s  Stephen Bannon and (more recently) Citizens United’s  David Bossie. The press has focused on Breitbart’s fractious role in the conservative world, but Mr. Bannon and Mr. Bossie have both devoted careers to tracking and exposing Mrs. Clinton’s ethical troubles.

Turning voters away from Mrs. Clinton is the groundwork. Mr. Trump still needs to give voters a reason to turn toward him. His sober approach of recent weeks is a start. A challenge will be to translate what has so far been campaign-engineered press releases and speeches into an on-the-fly prosecution of Mrs. Clinton during the presidential debates. Mr. Trump is right that Hillary is unfit to be president. Now, to keep proving it.