Talks between US east coast shipping companies and dockworkers broke down on Wednesday - less than two weeks before their labour agreement expires.
Retailers now fear that a strike is inevitable and the National Retail Federation wrote to President Barack Obama this week to ask him to use "all means necessary" to head off action.
But International Longshoremen's Association spokesman James McNamara said the union knows what is at stake, but must protect its membership.
"We offer the labour that keeps commerce moving," he said.
"If management doesn't appreciate or respect the labour that has made them a lot of money, we have to do what we have to do."
The longshoremen's union represents 14,500 workers who handle shipments from Maine to Texas - about 110 million tons.
Bosses and the union can't agree on container royalties.
The royalties are payments to workers based on cargo weight. They were created in the 1960s to boost wages after automation cut salaries and jobs.
The container carriers and port operators want to cap the royalties at 2011 levels, claiming they hurt the industry's competitiveness.
But the union says the payments aren't a bonus, they're an important supplementary wage.
It argues that management previously agreed to remove the royalties cap in exchange for being allowed to use $42m (£25m) of royalty payments to cover a wage increase.
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