Northwest already has permission from a bankruptcy judge to impose an earlier tentative agreement rejected by 80 percent of flight attendants in June. "We are now implementing new contract terms and conditions," said Mike Becker, senior vice president of human resources and labor relations, in a statement.
The union said that was its trigger for random, unannounced strikes.
"As soon as there are unilateral changes to the contract, that's our go-ahead," said Association of Flight Attendants spokeswoman Corey Caldwell.
The airline has said any strike would be illegal and that it would seek a court order to block one.
The vote was 55 percent against the new contract and 45 percent in favor.
"This decision is an example of the flight attendants' determination not to watch their livelihoods be squandered by management," she said.
Flight attendants faced pay cuts of roughly 21 percent, and the union said the reduction amounted to 40 percent once health care costs and other givebacks were factored in.
"Our members have spoken: these drastic cuts to our pay, benefits and work rules are simply unacceptable," said a statement from Mollie Reiley, interim president of the union's Northwest branch.
The union said it hoped Northwest would negotiate more. But Northwest, which has been seeking worker pay cuts since 2003, said time was up. Its new agreements with pilots and ground workers cannot take effect until it has a new contract with flight attendants, too either one that workers approve, or one the company imposes on them.
"Notwithstanding the results of the flight attendants' contract vote, Northwest must continue to move forward with its restructuring efforts," said Mike Becker, senior vice president of human resources and labor relations.
He said the imposed contract would save the airline $195 million, the same as the other two tentative agreements rejected by flight attendants.
If flight attendants walk off the job it would be Northwest's second strike in less than a year. Union mechanics struck in August 2005 rather than accept pay cuts. Northwest replaced them by hiring replacements and sending their work to outside contractors.