* Stalemate fuels speculation of possible lockout of workers
* Coast Guard prepares for potential waterfront labor unrest
* Union calls on grain shippers to return to bargaining table (Adds details, background, byline)
By Laura L. Myers
SEATTLE, Dec 24 (Reuters) - Dockworkers at four ports in the U.S. Pacific Northwest voted overwhelmingly to reject a "final" contract offer presented by grain shippers, their union said on Monday, setting the stage for a possible labor clash that could hamper exports from the region.
The stalemate in contract talks, and management's failure to win approval of its offer, fueled speculation that the shipping companies might move to impose a lockout of union members in a bid to keep grain terminals operating with replacement workers.