Congressman Mike Honda and his staff appear to have improperly used U.S. government resources to benefit his re-election campaign and linked official activities to political donations, the Office of Congressional Ethics said Thursday as it released a long-awaited report.

The pattern of allegations represented the most serious charges leveled against a Bay Area House member in recent memory.

The independent nonpartisan office’s 41-page report, presented to the House Ethics Committee on June 5 but not made public until Thursday, details a series of instances in which Honda’s staffers — particularly Chief of Staff Jennifer Van der Heide and former District Director Meri Maben — blurred or crossed the line between their official duties and working for Honda’s campaign.

That’s forbidden by federal law as well as House rules and standards of conduct. The Ethics Committee — evenly split between Republicans and Democrats — announced Thursday it will keep reviewing the matter, a process for which there is no deadline.

This “does not itself indicate that any violation has occurred, or reflect any judgment on behalf of the Committee,” Chairman Charles Dent, R-Pennsylvania, and ranking member Linda Sanchez, D-Lakewood, cautioned in a joint statement.

But the ethics board’s unanimous recommendation for further investigation indicates there could be a long, ugly road ahead for the veteran Silicon Valley politician.

The accusations arose last fall as Honda, D-San Jose, ran a nationally watched race against Democratic challenger Ro Khanna, a former Obama administration official, to win an eighth term representing a Silicon Valley district that is the first majority Asian-American House district outside Hawaii. A pair of locally elected officials who had endorsed Khanna filed the formal ethics complaint based on information provided by a former Honda staffer.

This probe is sure to be a major campaign issue as Khanna runs against Honda again in 2016.

Khanna campaign spokesman Hari Sevugan called Thursday “a sad day” for Congress and the 17th Congressional District given the “highly disturbing” findings: “We are hopeful that with subpoena power, the people can get to the full truth of the allegations that Mike Honda offered special access to his biggest donors and used his office for his own political gain.”

The report — based on a dozen interviews and about 1,400 pages of emails and documents provided by Honda’s office — says office staffers doubling as campaign volunteers apparently researched a potential campaign opponent, prepared campaign materials, used information from the congressional office for campaign purposes and regularly discussed campaign matters at official staff retreats and during “coffee breaks” in Honda’s congressional offices. Accounts from current and former staffers vary on whether these things occurred on the taxpayers’ dime.

According to the report, Honda told investigators he has attended his district office’s staff retreats, which he said were all official business. Then the investigators showed him notes from an October 2013 retreat at which campaign manager Doug Greven said that Honda’s Washington office makes policy, the district office arranges events and the campaign uses those events to raise money.

“It’s open to a lot of interpretation, but it doesn’t look good,” Honda acknowledged to investigators.

Honda on Thursday told this newspaper he had attended only one or two such staff retreats in recent years, not including the one at which Greven spoke.

Greven refused the ethics office’s request for an interview and declined to comment Thursday.

Honda said he has insisted that his current staffers cooperate fully with the Ethics Committee. But when asked whether he would encourage Greven to cooperate, Honda replied: “I think he has to use his own judgment.”

The Ethics Committee can issue subpoenas to compel testimony. And that’s entirely likely to happen, said Larry Gerston, a San Jose State University political science professor emeritus.

“The report is fairly meaty, if for no other reason than so many of the staffers in one way or another confirmed the general allegations,” he said. “They may view it as harmless or unintentional … but the fact of the matter is, by their own admission, the lines were crossed again and again.

“Some of this, I think, is sloppy stuff, but you kind of expect someone who’s been in office as long as he has to run a tighter ship,” Gerston added. “It seems to be something of a pattern.”

The report details how Van der Heide worked with a campaign official to set the guest list for a State Department event that Honda co-hosted at Santa Clara University in February 2013. She, Honda and campaign members told investigators that those guests were not solicited for campaign contributions, yet Van der Heide emailed a campaign official at the time asking “how are we doing to outreach to them for $?”

When investigators asked her what she meant by that — and why she included the campaign’s fundraising consultant on that email — she replied: “I don’t know.”

The ethics office report also describes a “1,000 Cranes” project to identify 1,000 donors to give $1,000 each. In an interview with Honda on Tuesday, he said this was his own, personal initiative on which only one campaign staffer ever collaborated, with no participation or use by his official staff. He said he wanted to find a way to reduce the amount of time he must spend “cold-calling” potential campaign donors in order to remain competitive.

“It’s soul-crushing,” he said. “I call it my ‘dialing-for-dollars dungeon.’ It’s not who I am.”

The ethics office’s report shows the idea was discussed at a September 2012 district-office staff retreat. Notes from that retreat obtained by investigators say “to work, it will require MH to use his personal touch … also will likely be transactional — i.e. help me with this visa for my grandma.”

On Thursday, when asked why this was discussed at his official staff’s retreat, Honda replied, “I can’t explain.”

Lawyers for Honda, Van der Heide and Maben sent a 13-page rebuttal to the Ethics Committee in July, arguing the report “addresses actions which either do not violate applicable ethics rules or, at worst, present narrow concerns.”

“Most importantly, the report clearly establishes that Representative Honda acted ethically and had no participation in, knowledge of, or reason to know about any of the allegations at issue,” the lawyers wrote. “It also demonstrates that Representative Honda established appropriate guidelines to separate office and campaign duties and that office staff took this separation of duties seriously.”

Honda told ethics office investigators and said again this week that he hadn’t known of any violations until they were revealed last fall as the former staffer, Ruchit Agrawal, leaked internal emails.

The congressman said this week that he was “pretty upset” and “disappointed” to learn of blurring of his office and his campaign, but that it had escaped his notice because he has “a very heavy schedule and I delegate a lot of the day-to-day activities,” a situation he “can only surmise” is typical of many lawmakers. “But like I said in the last campaign, I take full responsibility for that.”

He said he had his staff retrained on ethics rules and his own office policies, in addition to instituting a “bright-line” rule this summer that “no official staff may participate in my campaign — there’s a complete prohibition on that. … It’s probably the most aggressive policy that any member of Congress has.”

Van der Heide issued an apology last fall, saying that though she had volunteered for his campaign on her own time and didn’t use official resources, “I fell short of the congressman’s expectations and the example I try to set for the office.”

Honda said Thursday he has not punished her or anyone else and doesn’t intend to.

“I don’t feel that there’s anything I should be doing right now as to any kind of disciplinary action,” he said. “I still hold my sense of appreciation for the work that they’re doing and their efforts. I don’t feel it’s misplaced faith, I feel that mistakes were made and we addressed them.”

Josh Richman covers politics. Follow him at Twitter.com/Josh_Richman. Read the Political Blotter at IBAbuzz.com/politics.