Secretary of State Dennis Richardson has accused the Oregon Department of Justice of trying to unduly influence a civil investigation into a political deal between Gov. Kate Brown, labor unions and Nike.

An unsolicited letter of advice about the case from the deputy attorney general prompted Richardson to criticize the letter as inappropriate and "unprecedented" in a missive he sent Tuesday to Attorney General Ellen Rosenblum.

Richardson, the only Republican in statewide office, criticized the "apparent eagerness to influence this case" by the Justice Department and reminded Rosenblum of their collective duties to "rise above the political fray" and look into election complaints "without favoritism."

"Your conduct appears inconsistent with our shared task of impartial investigation, and we respectfully request that it never be repeated," Richardson wrote.

It is highly unusual for one statewide officeholder to criticize another publicly, even if they belong to different parties. The most recent notable instance was when then-State Treasurer Ted Wheeler called on Gov. John Kitzhaber to resign.

Richardson declined to comment. A spokeswoman for Rosenblum, a Democrat, forwarded a letter penned Wednesday by Fred Boss, the deputy attorney general whose original letter drew Richardson's ire.

"We did not ask or order you to suspend your investigation, nor was that our intent," Boss said.

Boss said the Justice Department issued the guidance because officials worried the Richardson would find an election law violation where there was none "and we would subsequently be unable to defend that finding."

"That would have benefitted no one," Boss wrote.

The agencies run by Richardson and Rosenblum have concluded no laws were broken when Brown, Nike and public employee unions brokered an apparent deal over an initiative petition. The initiative, sponsored by the unions, would have required some corporations to disclose tax information and other closely held business details.

Days before the deadline for initiative supporters to turn in signatures, Nike created a political action committee, called the Common Good Fund, and seeded it with $100,000. Shortly after, the unions abandoned their pursuit of a corporate disclosures initiative, despite having collected enough signatures to place it on the ballot.

The Common Good Fund has since paid $20,000 to another committee working to defeat Measure 105, which would repeal the sanctuary state law. That "no" campaign has the unions' support.

A Portland resident complained about the arrangement to the Justice Department, alleging it violated a law against paying to "sell, hinder or delay any part of an initiative, referendum or recall petition."

The Justice Department declined a criminal investigation in August and sent the case to Richardson's elections division for a civil investigation. Elections officials concluded there was no wrongdoing on Oct. 19.

But on Oct. 4, Deputy Attorney General Fred Boss sent a letter to Steve Trout, the state elections director, and Misha Isaak, the governor's state lawyer, concluding the initiative petition deal was a "political outcome." What took place between the governor, Nike and union leaders is free speech protected by the Oregon Constitution, Boss said.

In his letter, Richardson said he found the Justice Department's "insistence on providing unsolicited advice" odd given it has not resolved 55 voter fraud cases referred to it by Richardson last fall.

A spokeswoman for Rosenblum said Wednesday that the Justice Department's review of the cases is ongoing and has already resulted in one conviction.

-- Gordon R. Friedman