The odds that a new president in Washington will help the long-delayed Keystone XL pipeline become a reality took a hit on Tuesday.

Democratic presidential candidate Hillary Clinton announced in a town-hall-style meeting in Iowa that she was firmly against the project.

"I oppose it because I don't think it's in the best interest of what we need to do to combat climate change," said Clinton.

She took an even more hardline stance against the project in a Tweet on Tuesday evening.

"Time to invest in a clean energy future 鈥� not build a pipeline to carry our continent's dirtiest fuel across the U.S.," she wrote.

The move marks a stunning departure for Clinton, who is the frontrunner in most U.S. presidential election polls.

Previously, the former secretary of state said she was inclined to support the controversial pipeline.

Many supporters of the proposed 1,900-kilometre project were hoping a change in leadership at the White House could get the project a stamp of approval.

President Barack Obama, whose term ends in 2016, is widely expected to reject the project over environmental concerns.

And you can count Conservative Leader Stephen Harper among those who are likely disappointed by Clinton's announcement.

Harper has called the Keystone XL a "no-brainer" and suggested that a future president will eventually approve the project, and grant the necessary border permit.

On Tuesday, the Conservative Leader opted to pull his punches.

In a statement made to 麻豆影视, a spokesman for the Harper's campaign lauded the pipeline鈥檚 economic benefits, saying it would create jobs and bring security to Canada's energy sector.

"This is not a debate between Canada and the U.S.," said Conservative Party spokesperson Chris McCluskey.

"We know the American people support the project. We will not engage in presidential primary debates."

The news also sparked criticism from the Liberal Party, which has blamed Harper for failing to bring the project to fruition.

A spokesperson for Liberal Leader Justin Trudeau told 麻豆影视 that under Harper's leadership, the Conservative Party "failed to move the yardstick on one of the most important infrastructure projects of our generation."

"Mr. Harper has needlessly antagonized our closest friend and most important market, and the Conservatives have failed to gain access to the growing markets of the Asia-Pacific region," said press secretary Cameron Ahmad.

Trudeau has thrown his support behind the project because he believes it is in the public's interest.

Meanwhile, NDP Leader Tom Mulcair is opposed to the pipeline because he believes it will see 40,000 jobs exported to the U.S.

Instead, Mulcair would prefer to have the oil transported across Canada to keep the jobs in the country.

In a statement, the NDP stressed its fears over economic downsides of the deal and the potential environmental impact.

"Hilary Clinton's comments today reflect concerns on both sides of the border about this pipeline project," read the statement.

"Despite these concerns, Justin Trudeau has been cheering Stephen Harper along as he has ignored possible environmental consequences and thousands of job losses."

Environmental activists north of the border celebrated Clinton's decision.

"Stephen Harper's failure to act on climate means his 'no-brainer' pipeline is now supported only by the Donald Trumps of the world," said Mike Hudema, a spokesperson for Greenpeace Canada.

"It's time our politicians recognized that failing to address a growing climate crisis and trying to push high-carbon oil into a carbon constrained world is a losing proposition," he added.

The $8-billion TransCanada pipeline would see crude oil and bitumen transported from Alberta's oilsands through the U.S. to Nebraska. An existing network of pipelines would then carry the oil to refineries in the Gulf Coast.

Keystone would be responsible for transporting one-fifth of all of Canada's oil exports to the U.S.

Findings from the U.S. State Department have indicated that the pipeline is a cleaner alternative to rail, with Keystone creating 28 to 42 per cent lower greenhouse-gas emissions.

With files from The Canadian Press