Costco workers practice picketing at a warehouse in San Diego.TeamstersCostco workers affiliated with the Teamsters union have voted in favor of going on strike.The strike will start if no deal is reached by the January 31 deadline.Teamsters president Sean O'Brien has pledged to win "the strongest contract in Costco's history."Costco workers are ready to strike.Unionized warehouse employees affiliated with the Teamsters union have voted "overwhelmingly" in favor of going on strike, the union told Business Insider.The vote came ahead of a January 31 contract expiration deadline.