Mayoral candidate Ben Cayetano says he is consulting with attorneys on whether to sue to stop what he calls a smear campaign by a pro-rail interest group.

The issue arose after Pacific Resource Partnership, an advocacy group for the state’s unionized carpenters and construction workers, released an internal poll this week that showed fading support for the two-term governor.

The poll included a question that referred to Cayetano’s failure to pay back illegal campaign contributions made to his former gubernatorial campaign by exploiting a “loophole” in the law.

“I’m going to be talking to lawyers about it because I’ve got to put a stop to this,” Cayetano said Thursday.

John White, PRP executive director, defended the poll and its methodology as factual and provided documents that he said support his claim. He called on Cayetano to return more than $530,000 that was owed by his campaign committee when it shut down once he was out of office.

“We will not be silenced by bully tactics and threats of lawsuits,” White said. “We maintain that these illegal donations are directly tied to nonbid contracts awarded by the Cayetano administration.”

Cayetano is running as an anti-rail candidate against Mayor Peter Carlisle and former city Managing Director Kirk Caldwell, both rail supporters.

PRP’s poll raised the issue of money illegally donated to Cayetano and others before the state Campaign Spending Commission cracked down in the 1990s on the “pay to play” culture that existed for years between politicians and contractors in which many of those donors later received millions in nonbid government contracts.

The contributions linked to Cayetano involved donors using false names, said Gary Kam, general counsel for the Campaign Spending Commission. Once such contributions are discovered, the donor typically is fined, and the campaign is notified that it must “escheat” or forfeit the money to the state.

Cayetano paid back about $8,000 but closed his gubernatorial campaign still owing about $530,000. Once the campaign is closed, the commission can no longer go after the money because it is not considered a debt or a fine, Kam said, citing a 2006 attorney general’s opinion.

“It’s not a penalty,” Kam said. “The penalty is against the contributor.”

Kam said that other candidates who received false-name donations during the 1990s closed their campaigns also owing large sums.

PRP used the issue in its poll asking respondents to rate whether certain statements raised doubt in their mind about Cayetano.

White stood by the poll and said even though Cayetano may be clear legally, he should still return the full amount before raising funds for his mayoral bid.

“Morally, we believe he should pay it back,” White said, “because that money was done through illegal campaign contributions. To completely walk away from that obligation is not right.”

Cayetano said the tactic amounted to a “push poll” that was part of a larger smear campaign orchestrated by PRP aimed at discrediting him. Push polls involve calling large numbers of people and asking questions intended to influence voters while masquerading as a poll.

“It’s dirty campaigning is what it is,” Cayetano said.

Cayetano maintains his campaign did not know about the false-name donations until after the money had been spent. Kam said investigations into such donations can sometimes take up to two years. Meanwhile, candidates are free to raise and spend money.