The Pacific Resource Partnership, a consortium of union carpenters and private contractors that aimed a relentless barrage of negative advertising at former Gov. Ben Cayetano during his unsuccessful campaign for mayor in 2012, has agreed to apologize to Cayetano.

The message, which will be published Sunday in a half-page ad in the Honolulu Star-Advertiser, settles a defamation lawsuit Cayetano brought against PRP in the weeks before his election loss.

The full text of the message was not available Thursday, and Cayetano and PRP were limited in what they could publicly discuss about the settlement, but the former governor said it does contain an apology.

"I’m satisfied and I’m pleased with the agreement," he said.

Cayetano had claimed PRP’s ad campaign tarnished his reputation by unfairly portraying him as corrupt for accepting about $500,000 in illegal campaign contributions during the Democrat’s narrow re-election victory over Linda Lingle, a Republican, in 1998. Cayetano insisted he did not know the donations were illegal until after the money was spent by his campaign and the election was over.

Bob Watada, who had led the state Campaign Spending Commission’s investigation into "pay to play" schemes involving engineers and contractors seeking state and county contracts, said before the mayoral election that Cayetano had done nothing illegal.

John White, PRP’s executive director, declined to comment Thursday beyond confirming the settlement.

"We have reached a settlement that both parties are satisfied with," he said in a statement. "We have no additional comment on the matter."

While an apology offers some vindication for Cayetano, it is less clear what lesson, if any, the suit has for political speech in Hawaii. The legal standard for proving defamation in court is set higher for public officials, who generally have to show that false claims were made against them with actual malice — either that they were knowingly false or made with reckless disregard for the truth.

"I think it may make people a little more cautious in what they say," Cayetano said. "To make sure that what they’re saying is accurate, it’s correct."

Dan Boylan, a political analyst and former history professor at the University of Hawaii-West Oahu, said PRP’s ads were meant to undermine Cayetano’s strengths.

"You might not like him, but you believed he was a decent guy telling you straight up. Sometimes candid to a fault," he said.

The negative ads, Boylan said, "impugned the essence of who he is, put questions in some people’s minds about him, that maybe he’s not a man of integrity. Maybe he’s not a guy who plays by the rules. And I hope that organizations like PRP and others that we have seen and we will see again will take a step back.

"But I doubt that they will."

PRP’s political action committee spent more than $3.6 million in ads, opinion polls and other outreach against Cayetano, who opposed the city’s rail project, which PRP argues will provide jobs for union carpenters and contractors. PRP also invested an undisclosed amount of money — that sources privately estimate at well into the seven figures — on I Mua Rail, a multimedia ad campaign extolling the potential benefits of rail.

PRP also conducted a sophisticated field campaign that used polling and paid canvassers to target voters on behalf of Kirk Caldwell, who finished second behind Cayetano in the August primary but beat Cayetano 54 percent to 46 percent in the November runoff. The microtargeting strategy helped steer Democratic voters, regardless of their opinion on rail, toward Caldwell in the nonpartisan race.

Political columnists, editorial writers and many political analysts condemned PRP’s tactics. Even some labor leaders who support rail and who wanted to defeat Cayetano for his clashes with unions while governor complained privately before the August primary that PRP’s negative ads were ineffective.

Cayetano himself told the Star-Advertiser before the primary that he thought the ads had backfired: "It’s the impression I get from the street, anyway," he said. "I wish they spent another million dollars attacking me."

But the former governor had a darker view before the runoff. In a news conference announcing his defamation lawsuit, he blamed PRP for his flagging poll numbers.

"I feel frustrated, I feel humiliated," he said. "There are times I feel very, very bad for my family because my wife and my kids are hurt by these. It hurts."

In postmortems after the election, however, a more nuanced assessment emerged.

The Hawaii Poll had consistently placed Cayetano’s support in the mid-40s and never had him over 50 percent with voters. Some political analysts have credited PRP’s negative ads for helping to keep the former governor from breaking 50 percent and winning outright in the primary, but his poll numbers, according to the Hawaii Poll, never experienced a significant decline.

Many voters perceived Cayetano as a single-issue candidate on rail, and, while respected for his independence, he was not especially popular when he left office as a two-term governor a decade before.

Mayor Peter Carlisle’s decision to throw his support to Caldwell after finishing third in the primary, guidance from U.S. Sen. Daniel Inouye’s political network on Caldwell’s behalf, and PRP’s field campaign have been credited for pushing Caldwell over Cayetano in the runoff.

PRP has been the most visible example of a local independent expenditure committee — or super PAC — since the U.S. Supreme Court’s 2010 ruling in Citizens United v. Federal Election Commission, which allowed corporations and unions to spend unlimited amounts of money on politics as long as the spending is not coordinated with candidates.

PRP’s PAC, financed entirely by the Hawaii Carpenters Market Recovery Program Fund, did not have to disclose the names of individual donors to the fund. I Mua Rail, meanwhile, was not subject to campaign-finance disclosure requirements because rail was not on the ballot during the 2012 election.

State lawmakers, in response to Citizens United and after PRP’s aggressive spending, approved a law last year that requires super PACs to disclose the names of the top three donors behind ads as part of the content of the ads. The requirement will take effect after this year’s elections.