A senior aide to Rep. Colleen W. Hanabusa (D-Hawaii) told his colleagues late last month that the nation’s top drug lobby had agreed to run a campaign supporting the congresswoman’s challenge to Democratic Sen. Brian Schatz and wanted to coordinate it with her strategists.

Such an effort, described in an e-mail obtained by The Washington Post, could run afoul of campaign finance laws, which prohibit candidates and their staff from substantial discussions with interest groups about their independent political activities.

Officials with the Pharmaceutical Research and Manufacturers of America (PhRMA) and Hanabusa’s campaign denied that the group had offered to run such an effort but acknowledged talks about a possible fundraiser for Hanabusa and about the state of the race in general.

Campaign officials blamed the e-mail on a misinformed staffer.

“He made inaccurate assumptions about the type of help PhRMA could provide the campaign,” campaign spokesman Peter Boylan said.

Matt Bennett, a spokesman for PhRMA, said officials there did not offer to do a campaign on Hanabusa’s behalf. But he said the group had “preliminary” discussions about hosting an industry fundraiser for Hanabusa through its political action committee.

He also said that a PhRMA lobbyist had spoken with Jennifer Sabas, a top Hanabusa campaign adviser, but that they had talked only about the state of the Democratic primary campaign in Hawaii.

“They discussed the race and what’s happening on the ground,” Bennett said.

Boylan echoed that, saying Sabas did not provide PhRMA with any information “that would constitute coordination in violation of the law.”

But Clay Schroers, Schatz’s campaign manager, said the arrangement the e-mail outlines “is a deeply troubling situation, and Rep. Hanabusa clearly owes the people of Hawaii an explanation.”

The e-mail was sent June 28 from the Gmail account of Hanabusa’s deputy chief of staff, Christopher Raymond, to Sabas, Boylan and Rod Tanonaka, the congresswoman’s chief of staff. The Post obtained it from a person who received a copy and requested anonymity because of the sensitivity of the contents.

The message described a practice that is often suspected but rarely revealed: interest groups coordinating their putatively independent efforts with the candidates they are backing.

“As I’m sure you have heard, PhRMA has committed to pulling together an independent expenditure on CH’s behalf,” Raymond wrote. “Nick Shipley (Government Relations VP) and Bob Phillipone (Senior VP) are the leads on this and would like to be put in touch with folks on the campaign. After having talked with Nick about this a little more, and based on our discussion, I came to the conclusion that is it the three of you the he would like to be in touch with. I am going to give him your email address so he can be in touch. I didn’t feel comfortable giving out your phone numbers.

“Should you be contacted by Nick or Bob please know they are good democrats,” he concluded.

Boylan said that soon after the e-mail was sent, Raymond and other staffers were reminded that it was inappropriate to communicate with groups about independent expenditures.

“As soon as this issue was raised, it was shut down on the campaign and official office side weeks ago,” he said.

Hanabusa learned of Raymond’s e-mail a few days after it was sent and was assured that neither the campaign nor her congressional office had coordinated with the drug lobby, Boylan said.

Raymond, whose LinkedIn profile lists more than a decade of experience as a staffer in Congress and for trade groups, declined to comment.

Election-law experts said the kind of direct coordination described in the e-mail could violate campaign finance regulations if PhRMA ended up paying for an expenditure supporting Hanabusa’s candidacy. While groups can contribute up to $5,000 directly to a candidate through political action committees, they cannot have “substantial discussions” with a candidate or candidate’s staff about independent political activities.

“That most certainly, without a doubt, treads into very dangerous legal waters,” said Paul Ryan, senior counsel at the Campaign Legal Center, a nonpartisan group that advocates for stricter campaign finance rules.

Other campaign finance lawyers said it is difficult to prove illegal coordination, an area of the law that the Federal Election Commission has long struggled to define. Nonetheless, campaigns typically have strict rules barring interaction with independent groups.

“Generally, it’s best to steer clear of those conversations when conducting independent expenditures, but the devil is always in the details,” said Kenneth Gross, a former FEC associate general counsel.

PhRMA, one of the top lobbying groups in Washington, has not reported running independent expenditures on behalf of federal candidates in recent elections. However, it donated at least $8.3 million to politically active groups in 2011 and 2012, according to the nonpartisan Center for Responsive Politics.

Hanabusa shares PhRMA’s opposition to proposals that would require drug companies to provide rebates for medications that low-income beneficiaries obtain through Medicare.

Schatz, the incumbent senator whom Hanabusa is challenging, takes the opposite view. In April, Schatz co-sponsored legislation by Sen. John D. Rockefeller IV (D-W.Va.) to require such rebates.

“Senator Schatz stands by his position because it’s the right thing to do for Hawaii, and how that may influence outside groups is simply not a consideration for the senator,” Schroers said.

The two Democrats faced off in a previous House primary in 2006, when they both lost to Mazie Hirono, who now represents Hawaii in the Senate. The late Sen. Daniel Inouye named Hanabusa as his preferred successor before he died in December, but Hawaii Gov. Neil Abercrombie (D) selected Schatz, his lieutenant governor, to fill the remainder of Inouye’s term.