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., in a video posted early Tuesday to a union Facebook account.

The United States Maritime Alliance, which represents port employers, declined to comment early Tuesday. The two sides were not able to agree on wage increases, and the use of new technology in the ports was a sticking point for the union.

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.