THE SECOND-QUARTER campaign finance numbers for presidential candidates are out — but some of the most important information remains obscure. All candidates complied with federal election law by identifying individuals who contributed $200 or more to their campaigns. But none of the Republican candidates revealed the names of their “bundlers,” the big fundraisers who use their connections to raise hundreds of thousands of dollars. In other words, the identities of those to whom the candidates are most indebted remain hidden from the public.

Federal election law limits the amount an individual can donate to a campaign. This year, it’s capped at $2,500. That rule exists to reduce the influence one person has on a race. But a bundler collecting hundreds of thousands of dollars should ring similar alarm bells. The least candidates can do is to let the public know who’s underwriting their presidential bids.

A 2007 lobbying reform law required campaigns to report bundlers who are also registered lobbyists. Former Massachusetts governor Mitt Romney lists six. Requiring such disclosure makes sense, but why stop there? A corporate CEO who bundles checks for a candidate has the same potential interest in influencing a candidate.

As in 2004, President Obama’s campaign is going beyond what the law requires and voluntarily disclosing the names of anyone who raises more than $50,000. It further usefully disclosed names by the size of their haul, including an elite group of 27 who each brought in $500,000 or more. In past election cycles, Republican presidential candidates, including George W. Bush in 2000 and 2004 and John McCain in 2008, did likewise.

Yet this year, the Republican candidates have so far declined to publish the same type of list. Mitt Romney’s secrecy is particularly egregious, given that he understood the value of transparency sufficiently to disclose his bundlers in 2008. This cycle he has raised more than $18 million, four times the amount of any of his competitors. Indeed, he brags of having vacuumed up more than $10 million in a single day. How did he do this? By tapping bundlers, of course. But he won’t tell us who they are.

Asked why not, campaign spokeswoman Andrea Saul said, “We disclose all of the information about our donors as required by law, and anyone interested can review it publicly.” But what has changed since 2008? Ms. Saul did not respond.

A candidate should realize that it’s not only right to let the country know who’s propelling him to the White House—it could also be politically beneficial. Doing so would speak to a would-be president’s commitment to maintaining government openness once in the Oval Office.

The GOP recalcitrance — particularly striking in a party that claims to favor disclosure over campaign finance limits — is a reminder that Congress should require candidates to name their bundlers. But for now, Mitt Romney and the rest ought to live up to the standards set by George W. Bush, John McCain and, well, Mitt Romney.