U.S. President Barack Obama pointed to the global climate implications of approving TransCanada's Keystone XL pipeline during a wide-ranging Capitol Hill meeting on Wednesday with Democratic lawmakers.

Facing tough questions from some of his congressional allies about an apparent reluctance to approve the project, Obama called the debate a "global issue," Maryland Sen. Ben Cardin told reporters following the meeting.

"He pointed out this is a global issue, even more so than an American or a Canadian issue," Cardin said.

Obama has already insisted publicly that Keystone XL must not be approved if it's found to "significantly" increase global greenhouse gas emissions.

But in remarks that have sent chills down the spines of Keystone proponents in the U.S. capital, the president has also low-balled the number of jobs that might result from the $7 billion pipeline, which would carry millions of barrels a week of Alberta oilsands bitumen, and crude extracted from North Dakota's Bakken shale, to Gulf Coast refineries.

Job creation numbers vary

In a New York Times interview published over the weekend, Obama said Keystone XL would create "maybe 2,000 jobs during the construction of the pipeline," and added with a chuckle that it would sustain "somewhere between 50 and 100 jobs in a economy of 150 million working people."

That's in direct opposition to the U.S. State Department's draft environmental analysis of Keystone XL, which found it would support 42,100 jobs during the one- to two-year construction period, although only 35 permanent and temporary jobs would remain once it's fully operational.

The pipeline's proponents have long touted its job creation potential. Earlier this week, the president of North America's Building Trade Unions issued a statement rebuking Obama.

"The southern portion of the Keystone XL pipeline, which did not require a presidential permit yet was championed by the president and is currently under construction, has already produced millions of work hours and provided a positive economic benefit for the local communities," Sean McGarvey said.

Sen. Mary Landrieu, a Louisiana lawmaker who's one of several pro-pipeline Democrats in the upper chamber, put the president on the hot seat, urging him to approve Keystone XL and questioning why his recent public comments suggest he's opposed to the project. The pipeline has bipartisan support in the Senate.

'A healthy skepticism'

"He was very careful about not telegraphing his decision," Sheldon Whitehouse, one of the Senate's most passionate climate hawks, said following the meeting. "But he showed a healthy skepticism about the exaggerated economic arguments supporting it."

Landrieu and a fellow senator, Republican John Hoeven of North Dakota, introduced a resolution in the Senate later on Wednesday that declares Keystone XL in the national interest of the United States and urges Obama to approve it.

"We have studied this project for long enough," Landrieu said on the Senate floor.

"We know its tremendous economic benefits and the critical role it will have for our nation's energy security. With Nebraska now on board, every state involved has realized this project is a job creator with minimal effects on the environment."

Hoeven, meantime, criticized Obama's remarks to the New York Times that Canada could be "doing more" to reduce the oilsands' carbon footprint, suggesting such a condition was beyond American jurisdiction.

"After five years of delay, the president is talking about adding new requirements to a project. He's talking about adding those requirements in another country," Hoeven said.

Pipeline comments under fire

A group of senior Republican lawmakers also sent Obama a letter on Wednesday suggesting the president was jeopardizing the Canada-U.S. relationship with his recent comments on the pipeline, in particular his low-ball job estimate and his insistence that Keystone XL must not get the green light if it boosts greenhouse gas emissions.

"We are concerned that your recent statements have signaled an arbitrary and abrupt shift in how our nation approves cross-border energy projects," wrote Fred Upton, Ed Whitfield and Lee Terry, all members of the House of Representatives' energy and commerce committee.

"What was once a standard, apolitical process for approving pipelines with an allied friend and neighbour in Canada, a country with which we have a decades-long Free Trade Agreement, has now become an embarrassment."

The lawmakers asked Obama to clarify his job estimate, and to explain how it would be determined whether the pipeline results in increased carbon emissions — by the State Department, or by the White House.

"In your determination of whether the project will or will not 'significantly exacerbate the problem of carbon pollution,' are you taking into account the benefits of transporting crude oil by pipeline compared to other modes of transportation?" they asked.

"Will this standard be applicable to all cross-boundary energy projects that presently need presidential permits?"

The Republicans also requested a meeting with Obama to discuss the pipeline further.

The U.S. State Department is currently reviewing the pipeline because it crosses an international border. A final decision on Keystone XL isn't expected until late this year or early in 2014.