For the first time in nearly 50 years, longshoremen on the East and Gulf Coasts went on strike Tuesday, a move that will cut off most trade through some of the busiest U.S. ports and could send a chill through the economy.

“Nothing’s going to move without us — nothing,” said Harold J. Daggett, the president of the International Longshoremen’s Association union, addressing picketers outside a port terminal in Elizabeth, N.J., early Tuesday.

The United States Maritime Alliance, which represents port employers, did not immediately respond to requests for comment.

Businesses now face a period of uncertainty. Trade experts say that a short strike would cause little lasting damage but that a weekslong stoppage could lead to shortages, higher prices and even layoffs.

“When we talk about a two- to three-week strike,” said J. Bruce Chan, a transportation analyst at Stifel, a Wall Street firm, “that’s when the problem starts to get exponentially worse.”