Obama's order encourages government agencies to use PLAs on projects costing at least $25 million, but unions said it also directs his administration to report back to him on whether he should go further and require PLAs for federal contracts. 

Unlike each of the other 10 orders Obama has signed so far in his young administration, the president signed this one with no fanfare and out of the public eye.

"This executive order encouraging all federal agencies to adopt discriminatory, union-only project labor agreements is a shameless giveaway to Big Labor, which spent over a billion dollars to get Obama and pro-forced-unionism Democrats elected last year," said Stefan Gleason, vice president of the National Right to Work Legal Defense Foundation.

Among his 11 executive orders so far, Obama has signed four that aid unions, part of a union-friendly blitz in his early days.

Some observers said he's trying to do all he can for unions now because Democrats are unlikely to be able to deliver on unions' top priority, the Employee Free Choice Act, which would allow unions to form without a secret-ballot vote by workers.

Opponents, though, said the agreements can increase costs by as much as 20 percent because of higher union wages. 

"Union-only PLAs drive up costs for American taxpayers while unfairly discriminating against 84 percent of U.S. construction workers who choose not to join a labor union," said Kirk Pickerel, president of Associated Builders and Contractors.  "All taxpayers should have the opportunity to compete fairly on any project funded by the federal government."