“The political fight about Keystone is vastly greater than the economic, environmental or energy impact of the pipeline itself,” said Robert N. Stavins, director of the environmental economics program at Harvard. “It doesn’t make a big difference in energy prices, employment, or climate change either way.”

Environmentalists who have been arrested outside the White House protesting Keystone say that extracting petroleum from the Canadian oil sands produces more carbon emissions than conventional oil production and that the pipeline will provide a conduit to market for the oil. But a State Department review of the project last year concluded that building the pipeline would not significantly increase the rate of carbon pollution in the atmosphere because the oil is already making its way to market by existing pipelines and rail.

Republicans promote the project as a major source of employment and an economic engine, but the State Department review estimated that Keystone would support only about 35 permanent jobs. Keystone would create about 42,000 temporary jobs over the two years it will take to build it — about 3,900 of them in construction and the rest are in indirect support jobs, such as food service. In comparison, there were 241,000 new jobs created in December alone. Over all, the jobs represented by Keystone account than for less one-tenth of 1 percent of the American economy.

“This pipeline has become a symbolic issue all out of proportion to reality,” said Robert McNally, the president of the Rapidan Group, a Washington-based energy consulting firm and a former top energy official in the George W. Bush administration. “Why is what ought to be a routine matter turned into an all-consuming Armageddon battle?”