ORLANDO — The A.F.L.-C.I.O., the nation’s largest federation of unions, has issued an apparent endorsement of the Keystone XL oil pipeline — apparent because it enthusiastically called for expanding the nation’s pipeline system, without specifically mentioning Keystone.

And while some union leaders said the federation’s stance stopped short of an official endorsement, the nation’s building trades unions — eager for the thousands of jobs the pipeline would create — issued a statement saying the A.F.L.-C.I.O.’s stance was a clear endorsement of the Keystone pipeline.

The labor federation’s embrace of the pipeline, even with some ambiguity, will give President Obama some political cover as he weighs whether to approve the pipeline, which would carry more than 700,000 barrels of Canadian crude oil each day to refineries on the Gulf of Mexico.

But the A.F.L.-C.I.O.’s move is likely to strain the alliances that organized labor has sought to build with the environmental groups that are battling the pipeline.