President Obama took fire this week on several fronts over the proposed Keystone XL oil pipeline, as environmentalists amped up pressure against the controversial project and House Republicans linked the pipeline's approval to a must-pass debt ceiling bill.

More than two dozen environmental and liberal groups wrote to Obama Tuesday warning him against making any deals on the oil pipeline between Canada and the Texas Gulf Coast just as House Republicans are insisting that they won't approve an increase in the nation's debt ceiling unless the pipeline is approved.

The letter was signed by executives of some major Democrat-friendly groups, including, the Natural Resources Defense Council, the League of Conservation Voters, Greenpeace and Public Citizen — suggesting the possibility of a political backlash if the Obama administration approves the 1,179-mile project.

The groups are concerned about recent claims by Canadian officials that they are considering new policies that would mitigate pollution related to global warming in the oil and gas industries. President Obama has said he would base his decision on the pipeline largely on whether it adds carbon to the atmosphere, and environmental groups fear that Canada's tighter standards could prompt Obama to approve the pipeline in exchange.

In an interview with the New York Times in July, the president hinted that efforts by the Canadian government to lower carbon emissions "will go into the mix in terms of [Secretary of State] John Kerry's decision or recommendation" on the project.

The State Department, which must approve the pipeline because it would cross an international border, is expected to reached a decision early next year after finishing an environmental review.

"Our rationale is simple. Building Keystone XL will expand production in the tar sands, and that reality is not compatible with serious efforts to battle climate change," the groups said.

Canadian Prime Minister Stephen Harper on Thursday said his government will continue to push for the $5.3 billion pipeline even if Obama rejects it, saying he "won't take no for an answer."

But Harper added he is hopeful the project's potential national security benefits to the U.S. will make it impossible for the Obama administration to reject it.

"The logic here is overwhelming," Harper said. "I remain an optimist that, notwithstanding politics, that when something is so clearly in everybody's interest — including our interest as Canadians but [also] the national interest of the United States — I'm of the view that it has to be approved."

Most Americans seem to agree. A new Pew Research poll released Thursday shows 65 percent of Americans support building the pipeline, though the same percentage of the public support a recent Environmental Protection Agency proposal to tighten greenhouse gas emissions from power plants.

Meanwhile on Capitol Hill, House Republican leaders say they will demand approval of the Keystone pipeline as a condition for passing legislation needed to raise the nation's self-imposed borrowing limit of $16.7 trillion, which it's expected to hit Oct. 17. Failure to pass that legislation would result in the nation defaulting on its financial obligations.

White House press secretary Jay Carney on Thursday rejected the House GOP's Keystone demand, saying the president won't negotiate over raising the debt limit.

The Keystone provision in the Republican bill also has put some Senate Democrats who support the pipeline in a politically precarious spot. At least three — Mark Begich, of Alaska; Max Baucus, of Montana; and Joe Manchin of West Virginia — the pipeline should not be linked at all to debt ceiling talks.

“I’ve supported Keystone, but we should have a clean debt-limit bill,” Begich said. “That’s been the traditional way, and it’s been very successful.”

Wire service reports were used in this article.