Samuel C. Stretton, a local lawyer who has defended indigent clients for decades, said he would file a lawsuit as soon as the contract is signed.
He said he would first ask the state Supreme Court to halt the administration's plans. If the justices decline to get involved, he said, he will move on to federal court.
"Something has to be done," Stretton said, predicting that hiring a private firm would cause havoc in the courts.
The drive to hire a private law firm has been led by Deputy Mayor Everett Gillison, who spent 22 years as a public defender.
His goal, he said, is to provide more resources for indigent clients, either through a nonprofit or a private law firm that would be able to keep investigators and social workers on staff.
"I'm not afraid to take a chance," he said. "I think we'll get it right."
Many of the objections to the administration's plans are centered on the fear that a private firm would put profits before clients, who often struggle with poverty, addiction, and mental illness.
"The temptation will always exist to move cases along . . . to save the expense of investigation, mitigation, hiring experts, all of which impact the bottom line," Defender Association chief Ellen Greenlee said in October.
Gillison disagreed, noting that the lawyers who now serve as conflict counsel come from private firms or solo, for-profit practices.
"I, of course, reject the notion that private all of sudden equals bad," he said. "Private lawyers do the work now. They don't put profit above doing the work properly."
Stretton, who sued the city courts in the mid-1990s seeking pay increases for court-appointed attorneys, plans to list himself and City Councilman Dennis O'Brien as plaintiffs.
In October, O'Brien presided over a hearing on the plan to hire a private firm. He followed up with a meeting with the mayor.
"I told him, 'I think this is flawed,' " O'Brien said last week. " 'I think we can do better, and I'd like you to reconsider.' "
The mayor did not take O'Brien's advice.
The administration intends to hire a newly formed conflict counsel firm headed by Daniel-Paul Alva. The firm is expected to begin serving as the city's conflict counsel in March.
The administration would not provide a copy of the contract because it has not yet been signed, but Alva's bid was for $9.5 million - about $1 million less than the city ordinarily pays in conflict counsel fees.
Critics have warned that Alva's bid paid scant attention to dependency cases in Family Court, where attorneys represent parents and children dealing with abuse and neglect and the issues of parental rights.
In recent years, conflict counsel have handled 22,000 to 27,000 cases, about three-fourths of them dependency matters.
Gillison said Alva had a "stellar reputation," and he was confident his plan would provide more resources for parents and children in dependency court as well.
O'Brien cited a December ruling by a federal judge in Washington state who found two small cities there had violated the rights of indigent defendants by hiring private law firms to serve as the public defender.
The judge found the law firms did such a poor job as to constitute a "systemic failure" - a "natural, foreseeable, and expected result" given the caseloads they were expected to handle.
"Who's going to fix this when it all falls apart?" O'Brien asked. "When this model fails, and I'm absolutely certain that it will, you're going to have wasted this $8 to $10 million to hire a private firm."
Gillison said that, like all contracts, Alva's one-year deal included performance measures.
"If it works, it works," he said. "If it doesn't work, we don't have to renew."