SAN FRANCISCO (Reuters) - Ports up and down the U.S. West Coast were ready to rumble back into action Wednesday as union longshoremen return to work after President Bush stepped in to stop a management lockout which has cost the U.S. economy billions of dollars.

The Pacific Maritime Association, which represents employers at 29 West Coast ports from San Diego to Seattle, was on Tuesday ordered to reopen dock facilities after President Bush invoked the Taft-Hartley Act to impose an 80-day cooling off period.

Port management officials said the ports -- which handle $300 billion of trade annually -- would gradually resume operations on Wednesday, with the first full shift starting at 6 p.m. Pacific (9 p.m. EDT)

It could take six to seven weeks to clear the backlog of cargo, which has been piling up just at the start of the crucial U.S. holiday shopping season, PMA officials say.

The 10,500-member International Longshore and Warehouse Union, which had argued against federal government intervention in its dispute with port management, said it was prepared to go back on the job.

But union officials have said that the huge backlog in cargo following the almost 10-day lockout had created congested conditions that might threaten worker safety -- hinting that "slow downs" might occur as longshoremen go back to work.

Some 200 ships carrying everything from auto parts to plastic toys to frozen food were sitting lined up off West Coast ports waiting to take on or unload cargo held up by the labor dispute.

WILL TAFT-HARTLEY WORK?

After months of unsuccessful contract negotiations that culminated in a management lockout on Sept. 29, Bush intervened in the dispute Tuesday, invoking the 1947 Taft-Hartley Act to obtain a court imposing an 80-day cooling off period on both sides.

Negotiations between the two will continue, and the port lockdown could resume if they emerge from the 80-day hiatus without a firm contract agreement.

The PMA and the ILWU, one of the best paid and most powerful unions in the country, had been at loggerheads over the issue of new port technology and union control over any new jobs that technology might create.

The union's last collective bargaining agreement expired on July 1. But negotiations to reach a new contract deal were rocky, and union leaders decided early last month not to extend the contract terms on the day-by-day basis which had kept ports running all summer.

Late last month port managers accused the union of staging illegal work slowdowns in port operations and ordered the lockout -- shutting down port facilities which normally handle about $1 billion in cargo per day.

Exporters, manufacturers, farmers and truck drivers quickly felt the financial pinch as the port shutdown cut off seaborne cargo traffic between America and some of its largest trading partners, raising fears of widespread repercussions in the faltering U.S. economy.

After marathon weekend talks between the two sides failed to reach agreement, Bush on Tuesday directed the U.S. Justice Department to seek the court order to reopen the ports, saying the situation had become so grave that threatened "a crippling effect on the national economy."

Labor groups have already criticized Bush's decision, which came less than a month before an election that will determine which party controls the U.S. Congress.

"It's not going to be forgotten by working people come the November elections," said Scott Kronland, a lawyer with the AFL-CIO, an umbrella labor group which filed a brief supporting the longshoremen against federal intervention.

By Andrew Quinn