A controversial proposal to build a massive underground pipeline to carry 700,000 barrels of crude oil per day from the oil sands of Alberta, Canada, to refineries in Texas has become the environmental issue of the summer, pitting developers and labor unions desperate for construction jobs against environmentalists and Native American tribes who fear the pipeline will spell environmental disaster.

TransCanada’s proposed Keystone XL project would consist of more than 1,700 miles of 36-inch-diameter pipe, about 327 miles of which would be in Canada while the rest would snake southward through the central United States. Because the pipeline would cross the international border between Saskatchewan, Canada, and Morgan, Mont., a special permit from the U.S. Department of State is required for the project to proceed.

More than 1,000 activists — including NASA climatologist James Hansen, who has urged the scientific community to “get involved in this fray” — are expected to descend on the White House starting Sunday for three weeks of civil disobedience and mass arrests. Six California activists are driving from Sacramento to Washington, D.C., as part of a “No Tar Sands Caravan” that leaves Sunday.

The American Petroleum Institute and the International Brotherhood of Teamsters, both of which are urging the State Department to approve the project, held a conference call with journalists Thursday in which they claimed the pipeline could generate 20,000 new jobs.

“Today, with the U.S. economy still struggling, nothing is more important than jobs,” said Cindy Schild, the petroleum institutes’s refining issues manager. “And construction of the pipeline would mean massive numbers of them.”

Jim Kimball, chief economist for the International Brotherhood of Teamsters, said that Teamsters President James P. Hoffa has written to Secretary of State Hillary Clinton urging approval of the project, which would create union jobs related to building and maintaining the pipeline.

“There are 14 million unemployed people in the United States and we think that big construction and infrastructure projects are a fast and good way to put a large number of people to work quickly,” Kimball said.

Several environmental organizations and Native American tribes are concerned that the pipeline, which will cross the Yellowstone River and the Ogallala Aquifer, could threaten drinking water of 2 million Americans if it leaks. And they say the process of extracting the tarlike oil, called bitumen, is energy-intensive and will destroy large parts of Alberta’s boreal forest.

The pipeline debate presents a thorny political issue for President Barack Obama, in part because labor unions and environmental organizations have traditionally been core Democratic constituencies. In an op-ed published this week in the Washington Post, author and environmentalist Bill McKibben urged Obama to make good on his campaign promise from 2008 that his election would mark “the moment when the rise of the oceans began to slow and our planet began to heal.”

“The issue is simple: We want the president to block construction of Keystone XL,” McKibben wrote. “We have, not surprisingly, concerns about potential spills and environmental degradation from construction of the pipeline. But those tar sands are also the second-largest pool of carbon in the atmosphere, behind only the oil fields of Saudi Arabia.”

The State Department, through its Bureau of Oceans and International Environmental and Scientific Affairs, is expected to issue a final environmental impact statement for the proposed pipeline sometime this month. That is followed by a 30-day public comment period and a 90-day review period during which other federal agencies, such as the Department of Energy and the Environmental Protection Agency, provide their input on whether issuance of a permit is in the national interest.

Karen Enger, 43, is one of the six activists taking part in a caravan from Sacramento to Washington, D.C. The group hopes that more drivers and riders will join the trip.

“The pipeline may not run through California, but it runs through our heartland,” Enger said. “And many of us in California voted for Obama and we want him to live up to what he said he would do to protect the climate during the campaign. We want Obama to stand up to Big Oil and say no to the pipeline.”

Contact Dana Hull at 408-920-2706. Follow her at Twitter.com/danahull.