Saskatchewan Premier Brad Wall and 10 U.S. governors have sent a letter to U.S. President Barack Obama, urging him to approve the proposed Keystone XL pipeline, which they say could carry up to "4 million barrels a day by 2020, twice what is currently imported from the Persian Gulf."

The letter was sent Thursday on letterhead from Wall's office. Featured alongside his signature were endorsements from the governors of Arizona, Kansas, North Dakota, South Dakota, Utah, Idaho, Nevada, Oklahoma, Texas and Wyoming.

"The energy relationship between the United States and Canada is vital to the future of both our countries. It is an interest we share, transcending political lines and geographic boundaries," the letter states.

Read the letter sent to Obama (PDF)

The proposed $7-billion pipeline -- which the letter argues is "fundamentally important to the future economic prosperity of both the United States and Canada" -- would carry oil from Alberta's oilsands to Steel City, Nebraska, and then on to refineries on the Gulf Coast.

Last year Obama reserved his decision on the controversial pipeline until after the election.

But as his second-term inauguration Friday draws nearer, advocates for the project appear to have run out of patience and are beginning to ramp up pressure.

The project, which has faced considerable opposition, is being proposed by TransCanada. On its website, the company says it intends to re-apply for the presidential permit needed to cross the border, and expects Obama to approve the application in the first quarter of 2013.

Speaking to CTV’s Power Play on Thursday, Wall said he is spearheading this latest push because “it goes to the broader issue of trade between our countries and the branding of Canadian exports, specifically the branding of Canadian oil.”

He added that should the pipeline be approved, one in seven barrels of oil in it “is actually American oil” from the Bekken formation, a rock unit that runs under parts of Montana, North Dakota, and Saskatchewan.

The letter outlines three key arguments for the pipeline's approval.

First, it states that the Keystone project would provide "crucial" energy security to the U.S., noting Canada's oil reserves are the third-largest in the world, behind only Venezuela and Saudi Arabia.

The letter also touts the residual economic benefits of the project, saying it is expected to create thousands of manufacturing and construction jobs, along with tax revenue and business activity in communities along the route.

Finally, the letter urges Obama to approve the project for reasons of efficiency and reliability, saying Wall and the governors are "committed to responsible stewardship of our shared environment."

"Pipelines remain the most efficient method of transporting large volumes of crude oil, but we also recognize the need to take the measures necessary to protect the environment and public health and safety."

According to Nathan Lemphers, senior policy analyst for environmental think-tank Pembina Institute, new findings indicate that any additional pipeline capacity developed in western Canada will increase oilsands production, causing an increase in greenhouse gas emissions.

“And it’s not small,” Lemphers said. “Filling the Keystone XL pipeline with oilsands will create the equivalent emissions of putting 4.6 million cars on the road.

“So at a time when we’re aiming to meet our climate targets that are set out under the Copenhagen Accord, Keystone XL will take us in the wrong direction.”

The proposed pipeline became a major issue during the U.S. election campaign, but Obama managed to sidestep it by putting any presidential decision on hold while a plan was worked out to keep the pipeline from running through Nebraska's sensitive Sandhills region.

Under pressure from opponents and even the governor of Nebraska, TransCanada revised the route, after earlier saying that wasn't an option, resetting the environmental review process.

Wall said he is “optimistic” that Nebraska Gov. Dave Heineman will eventually be more supportive of the pipeline.

Meanwhile, the company split the project into two, and began construction on a southern leg of the pipeline between Oklahoma and Texas while awaiting approvals on the northern section -- a sign of apparent confidence that the project's eventual approval is a fait accompli.