Updated

With President Obama poised to decide whether to allow construction of the Keystone XL Pipeline -- based on a second, more environmentally-sensitive path -- critics now appear focused on derailing the project over a climate change study.

TransCanada Corp. submitted a revised application after the president rejected the first one in January because it took the 1,700-mile-long pipeline across an aquifer in Nebraska.

However, environmental groups say producing oil from Alberta tar sands releases more carbon dioxide than conventional drilling, which would increase global warming, and that studies on the first application were inadequate.

Jeremy Symons, of the National Wildlife Federation, said Sunday the original assessment by the State Department underestimated the amount of greenhouse gas. And he called on the Environmental Protection Agency to do a more complete assessment on the second application.

“We need an honest reassessment, and it’s up to the president to look at all environmental impacts,” he told FoxNews.com.

Symons also suggested the State Department “ducked” the greenhouse emissions issue to avoid the politically sensitive debate about climate change.

The pipeline would transport the oil from Canada and Western states to Gulf Coast refiners.

Supporters argue the project would help the United State lessen its dependency on foreign oil, create thousands of construction jobs and that it has already passed the environmental test.

"After four years, the Keystone XL pipeline is perhaps the most exhaustively reviewed projects of its kind ever and already includes a climate review," Don Canton, spokesman for North Dakota Republican Sen. John Hoeven, said Sunday. "This recent tactic by opponents is just another attempt to slow or kill (the) project."

Canton also said 80 percent of all new development in the oil sands will have essentially the same carbon footprint as conventional drilling.

Hoeven is part of a bipartisan group of 18 senators that has asked Obama to meet with them over the issue. They say the pipeline could also carry 830,000 barrels of oil daily -- 100,000 of which are from North Dakota and Montana.

Nebraska environmental officials and the State Department must complete separate analysis before Obama makes a final decision.

The State Department has signaled that it would make a decision in the first few months of 2013.

Exactly which side now has the political upper hand is hard to gauge following Obama’s re-election last month, with polls showing mixing findings.

Environmentalists say the president campaigned again on clean energy and must fulfill his promise.

“Drilling in tar sand takes us in the opposite direction,” Symons said.

The first analysis concluded there would be fewer emissions in the United States without the pipeline because crude-oil production releases less carbon dioxide, according to Bloomberg News.

TransCanada’s Alex Pourbaix told Bloomberg he didn’t expect the State Department to make significant changes on how it views Keystone’s impact on climate change.

He said one reason is the region is going to get developed regardless of whether Keystone XL is approved.