Allina Health hospital nurses voted Monday night to reject a contract offer from their employer, increasing the likelihood that their walkout over health benefits, staffing and safety concerns will go down as the longest nursing strike in Minnesota history.

While the Minnesota Nurses Association had not recommended a “no” vote, many nurses said they felt Allina’s latest offer was too similar to one they rejected in August, and to the terms their union negotiators rejected during last-ditch negotiations in September to avert a strike.

A new sign reading “New Lipstick, Same Pig” appeared at the picket line outside Allina’s Abbott Northwestern Hospital in Minneapolis Monday morning, foreshadowing the vote result that the union announced at 10:30 p.m. in St. Paul.

While she declined to provide exact results, MNA executive director Rose Roach called the vote margin “resounding” and said it sent a clear message from front-line nurses to go back to the bargaining table. “Each of them voted with their conscience, and with their patients and their families in mind,” she said.

The results mean that strikes will continue at Abbott as well as United Hospital in St. Paul, Mercy Hospital in Coon Rapids, Unity Hospital in Fridley and the Phillips Eye Institute in Minneapolis.

More than 4,000 nurses have been on strike for 29 days, since Labor Day, after a one-week walkout in June. The state’s longest nursing strike, in 1984, lasted 38 days.

A written statement from Allina spokesman David Kanihan expressed a willingness by hospital executives to return to negotiations as soon as possible, but disappointment at the vote result.

“This proposal was eminently fair and went very far in addressing the issues the union raised during negotiations,” the statement said. “We are disappointed that our nurses will remain on strike instead of returning to the bedside to care for patients.”

When negotiations started in February, Allina insisted that the nurses give up their union-backed health insurance plans, which many nurses prefer because they feature low or no deductibles and provide protection against the frequent illnesses and injuries that come with nursing jobs.

It was an unorthodox step, given that Allina usually negotiates with its nurses in lockstep with competing hospital systems, including Fairview and HealthEast. Those systems quickly settled contracts with their nurses last winter, leaving health benefits untouched.

Union negotiators refused Allina’s demand until September, when they agreed to move all nurses by 2020 to Allina’s three corporate health plans, but they asked in exchange for some oversight of the corporate plans to ensure their cost and quality.

The latest Allina offer did not grant the level of oversight that the union sought, but it did agree to leave two of the union health plans untouched through 2019. The offer also guaranteed that the actuarial value of Allina’s most popular health plan would never change by more than 7 percent in any three-year contract period.

Amid rising hospital violence, the contract proposed to increase safety and de-escalation training for nurses and guarantee 24-hour security in Allina emergency rooms.

The union wanted charge nurses to be freed of direct patient assignments so they could support other nurses when their patient demands grew overwhelming. Instead, the contract offered to address such staffing concerns with a joint nurse-hospital committee — and with the involvement of a mediator when the two sides couldn’t agree.

The union had recommended that nurses vote no on two prior contract proposals earlier this summer, and the results of voting were strong enough that they led to the one-week strike in June and the current, open-ended strike.

Union officials were less certain of Monday’s result, because they made no such recommendation and asked their roughly 4,800 nurses to vote in the interests of their families and their profession.

Striking nurses lost their health insurance as of Saturday, and it was unclear how heavily that would influence the voting. Allina also has reported that 595 nurses have crossed the picket line and returned to work. While the union questions that number, it acknowledged that any such nurses still were able to vote on the latest offer.

The union filed claims with the National Labor Relations Board that Allina had engaged in “unfair” negotiating practices, such as declining to provide requested financial information. These claims provide striking nurses with federal protection against being permanently replaced.

Allina has kept its hospitals running by hiring more than 1,000 temporary nurses from across the country. Such nurses come at premium wages, though, which is why the two strikes have easily cost Allina more than $40 million so far.