Details of the program - more than three years in the making - are still being worked out. But according to industry officials who have discussed it with the Transportation Security Administration and Coast Guard, undocumented immigrants and people convicted of certain crimes might be barred from the positions they now hold.

At ports, thousands of people could be out of jobs, including dock workers and truck drivers.

''Of course there are concerns,'' said Chuck Carroll, executive director of the National Association of Waterfront Employers, a trade group for terminal operators. ''You'd have the same number of boxes but fewer people to move them, and that could mean major congestion.''

Steve Stallone, spokesman for the International Longshore and Warehouse Union, said a conviction shouldn't automatically preclude someone from working.

''Just because a guy got into a bar fight does not make him a terrorist,'' said Stallone, whose union represents nearly 14,000 West Coast longshoremen and clerks.

''Terrorist acts are one thing. But that you beat up your next-door neighbor? I don't think so.''

The issue in question is the Transportation Worker Identification Credential program, which seeks to better control access to harbors, rail yards, airports and other cargo transit areas terrorists might target. It could affect as many as 6 million people.

TSA and Coast Guard officials have refused to discuss details of the plan before it is unveiled, which could be as early as next week. The proposal would bar anyone who is on a terror watch list, entered the country illegally or has certain criminal convictions. In some cases, workers could be excluded for assault with intent to murder, kidnapping, rape, drug offenses, extortion, robbery and fraud.