“BIG BUSINESS, elite media and major donors are lining up behind the campaign of my opponent because they know she will keep our rigged system in place,” Donald Trump declared at the Republican National Convention last month. “She is their puppet, and they pull the strings.”

Never persuasive, this line of attack was at least less hypocritical when Mr. Trump largely self-funded his campaign. These days he is raking in donations — $80 million last month, according to the campaign — and from bigwigs as well as small donors. Among others, New York Jets owner Woody Johnson and coal executive Joe Craft are fundraising for the GOP nominee. So who, to use his formulation, is now pulling Mr. Trump’s strings?

Voters could get some answers if Mr. Trump would release, as have Hillary Clinton and most recent candidates before her, the names of his bundlers. These are the people who solicit large numbers of often medium-size checks and hand them over to campaigns. Though they do not necessarily give huge amounts of their own cash, bundlers rake in big sums for their candidates — and therefore pose a similar threat to the integrity of the political process. Even so, the law requires campaigns to disclose bundlers only when they are registered lobbyists. It is left to the campaigns to voluntarily provide more information.

Ms. Clinton has done so, releasing the names of some 500 bundlers. Mr. Trump, Politico’s Shane Goldmacher recently noted, has not. Nor would the campaign discuss with Politico, or with us, its intentions with regard to this rudiment of transparency.

Such contempt for voters is not surprising. Mr. Trump also is thumbing his nose at the long tradition of major-party presidential nominees releasing their tax returns. He promised to do so. Ms. Clinton has released decades of hers, including her 2015 return, which she disclosed Friday . Yet Mr. Trump continues to keep his “beautiful” tax documents under wraps.

Why the secrecy? With respect to bundlers, Mr. Trump might not want to draw attention to the special interests now backing him, and some of his bundlers might (understandably) be embarrassed to be outed. With respect to his tax returns, the candidate may not want to reveal that his business is not as successful as he claims, that he is stingy with charity or that he has compromising business relationships in Russia or elsewhere overseas.

No matter the reasons, Mr. Trump’s refusal to meet essential standards of transparency expresses contempt for the democratic process and erodes crucial norms. If voters reject him in November — and his secrecy provides yet another reason they should — we hope future presidential candidates will conclude that these traditions are not so easily discarded.