The proposed Keystone XL pipeline has been a 1,179-mile headache for President Obama.

The project has divided Democrats, with environmentalists fighting the proposed pipeline that would carry Canadian crude oil from Alberta's tar sands to refineries on the Gulf Coast, while labor groups and some Democratic lawmakers from oil states want the jobs the project would create.

But as the State Department conducts its final evaluations on the proposal, the president has appeared increasingly reluctant to sign off on the long-delayed pipeline without more assurance it wouldn't hurt the environment. His position has pushed the sides into overdrive in an attempt to woo him before it's too late.

"We are never overly confident. After you've been in a fight for four years and you know folks' land is on the line, you never let your guard down," said Jane Kleeb, executive director of Bold Nebraska, an advocacy group opposed to the pipeline. But "we certainly feel a lot more positive about the president denying the pipeline today than we did even two months ago."

The State Department, which must approve the pipeline because it would cross an international border, says there is no timetable for its decision. But a department official speaking on background said a ruling could come by the end of the year.

The president says he will evaluate the $5.3 billion project, proposed by the Canadian company TransCanada, based on whether it would significantly add carbon to the atmosphere, which scientists say contributes to global warming.

"Allowing the Keystone pipeline to be built requires a finding that doing so would be in our nation's interest. And our national interest will be served only if this project does not significantly exacerbate the problem of carbon pollution," he said in a June speech at Georgetown University in Washington.

In an interview with the New York Times last month, the president said Republican claims that Keystone would generate tens of thousands of jobs were inflated.

"There is no evidence that that's true," he said, adding that "realistic" estimates shows construction likely would create about 2,000 construction-related jobs.

"That is a blip relative to the need," he said.

Optimism among Keystone opponents has been further buoyed by a recent decision by the State Department's independent watchdog to review whether the contractor the department hired to conduct an environmental study had a conflict of interest.

The report, by Environmental Resources Management, suggested the pipeline would have little effect on the environment or global warming. But liberal publication Mother Jones reported that analysts who helped draft the report had worked for TransCanada and other energy companies poised to benefit from the pipeline.

The State Department's Inspector General's office said it has "initiated an inquiry" into the claim. While the watchdog won't say how long it expects the review to take, the agency likely will hold off on a decision until after the report is done.

A September court case involving a lawsuit by three Nebraska landowners who would be affected if the pipeline is built could delay the project further.

A win for the landowners, who oppose the pipeline, likely would force TransCanada to resubmit its plans with the U.S. government -- which could set the project back years.

Meanwhile, some unions have backed the project, citing its potential economic impact.

"The Washington politics behind the delay of the Keystone XL pipeline are of little concern to those seeking the dignity of a good, high-paying job," said Terry O'Sullivan, president of the Laborers' International Union of North America.

Republican and Democratic lawmakers from oil-producing states also have pressed the administration to approve the pipeline. Sens. Mary Landrieu, D-La., and John Hoeven, R-N.D., have submitted a joint resolution in Congress calling the pipeline a "national interest."

"We have studied this project for long enough," Landrieu said. "We know its tremendous economic benefits and the critical role it will have for our nation's energy security."

In the Republican Party's weekly address, Sen. Tim Scott of South Carolina said Obama "is so out of touch with unemployed Americans that he thinks tens of thousands of Keystone XL construction jobs are a 'blip, and 'not a jobs plan.' "

The Canadian government for its part has spent millions of dollars marketing the pipeline through its "Go With Canada" campaign. The effort plays up the two countries' friendly relationship, calling it a "a model for the world," while promoting Canada as a "world environmental leader" in the field of oil and gas development.

Secretary of State John Kerry, who was considered one of biggest climate-change advocates while in the Senate, has been mostly quiet about Keystone since moving to the State Department in February. His silence has both sides speculating how much input and influence, if any, he will have on determining if the pipeline gets built.

"It's baffling he hasn't said anything about it," Kleeb said. "We feel that the president and his team are engaging, but Secretary Kerry is a little bit aloof on the issue."