After years of hit-and-miss labor activism at Amazon.com Inc., the International Brotherhood of Teamsters has adopted a more direct tactic: calling for a nationwide strike six days before Christmas.

The union asked workers to walk off the job Thursday morning at seven Amazon facilities where the Teamsters have sought to represent the company’s contract delivery drivers or warehouse workers. They’ve also pledged to dispatch members who don’t work at Amazon to set up picket lines at hundreds of warehouses around the country, their biggest show of force to date in a campaign to organize America’s second-largest private employer.