Canada's Natural Resources Minster Greg Rickford was on Capitol Hill again Tuesday pitching Keystone XL, as Senate supporters of the controversial pipeline tried to round up sufficient votes to override U.S. President Barack Obama's threatened veto of any Congressional attempt to force approval of the project to ship Alberta oil sands crude to the Texas Gulf coast.

After meeting Senator Joe Manchin, a West Virginia Democrat and Keystone XL backer, Mr. Rickford said: "I thanked Senator Manchin for his outspoken support of the Keystone XL pipeline project, and Canada as the partner of choice to fulfill the U.S. increasing need for crude imports."

But Mr. Manchin's backing comes with conditions the Canadian Conservative government and TransCanada Corp., the pipeline giant seeking permission to build the $8-billion (U.S.) project, may find tough to accept.

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The senator – who believes a veto-proof majority can be achieved – says he plans to back amendments proposed by some of his Democratic colleagues. They would ban the export of oil carried by Keystone XL, require that all future purchases of pipe for the project be bought from U.S. steel makers and force payments into the national oil-spill fund from which Canadian oil sands crude is currently exempt.

Many Republicans oppose those amendments and none may survive into the final bill. Nor do any of them appear in the House of Representatives Keystone XL approval bill passed last week.

All 54 Republicans in the Senate back Keystone XL, but the Canadian government and other advocates of the project need 13 more votes – meaning 13 Democrats – to forge the two-thirds majority of 67 in the 100-seat Senate needed to override a presidential veto.

Achieving that seems unlikely. Currently, nine Democrats back the legislation that would wrest control of the decision from Mr. Obama, who has repeatedly delayed deciding on the project, which opponents claim will massively add to greenhouse gas emissions by spurring extraction of Alberta's vast bitumen reserves.

No final vote on the Keystone XL approval bill is expected in the Senate until at least next week.

But in the first full day of debate Tuesday, the bitter and mostly partisan battle over Keystone XL was waged in terms of its geopolitical significance and whether it would hasten global warming.

Giving Keystone XL the go-ahead would allow the United States to reduce imports from places such as Russia, Venezuela and Saudi Arabia, Mr. Manchin said, echoing a refrain long made by the Canadian government, which has painted other foreign suppliers as mostly unreliable and hostile to the United States.

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In contrast, he lauded Canada as "the most stable regime, the best ally we've ever had."

Alabama Republican Senator Jeff Sessions was similarly pro-Canadian. There's "no better place" to import oil than from "Canada, our friend and neighbor," he said. Approving Keystone XL, which, when built, could deliver 830,000 barrels a day, would create "an additional supply from an ally of the United States that will bring down the price of oil," he said.

But opponents continued to portray Keystone XL as a project that would benefit Canadian coffers and worsen global warming.

Allowing Keystone XL to proceed would send "the dirtiest oil in the world, from the tar stands in Canada to a tax-free export zone so it can be exported," Massachusetts Democratic Senator Ed Markey said. "That oil should not come to our country, go right through it and out again."