But Prentice says he is encouraged that the controversial line is gaining increased bipartisan support in the halls of the U.S. lawmakers.
Prentice says he plans to travel to Washington in January, but no specific dates have been set.
The Keystone bill failed by one vote, just days after the House of Representatives approved the pipeline.
A final OK is still in the hands of U.S. President Barack Obama.
The pipeline would take Alberta bitumen to ports in Texas, but environmentalists say it poses a danger to the ecosystem and encourages the production of greenhouse gases.
"We're obviously disappointed that the Keystone project did not reach the requisite number of votes in the Senate," Prentice said Tuesday afternoon.
But Prentice is still hopeful it will pass eventually.
"It's fair to say that it's the end of a particular Congress and there is a new Congress that will be sworn in in the beginning of January, a new Senate, specifically, and I'm certain the process will carry on at that point in a Senate that is more clearly dominated by Republicans."
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