After the Keystone XL oil pipeline cleared an important hurdle last week, critics of the project are searching for ways to force more of the delays that have dogged it for more than four years already.

The State Department said Friday that TransCanada Corp’s pipeline from the Alberta oil sands to Texas would not add to global greenhouse gas emissions because oil sands crude will make it to market whether or not the project is built.

That interpretation neutralized a major argument that many environmentalists have put forward against the 800,000 barrel per day pipeline: that once built it would usher in greater development of the oil sands, where production is carbon-intensive.

The public has 45 days to comment on the State Department’s review. Once that step is taken the department has 90 days to determine whether the project is in the national interest, with a decision expected in August or later.

“We think 45 days is very insufficient given the 2,000-plus pages of analysis,” said Danielle Droitsch, head of the Canada Project at the Natural Resources Defense Council, one of the groups fighting Keystone. Greens will write letters to officials seeking to extend the comment period to 120 days, she said.

Environmentalists have reason to hope. In the years a decision has been pending on Keystone, the State Department has repeatedly delayed the process because of environmental concerns.

In mid-2010 State twice extended the comment period on an earlier environmental review after groups complained they did not have enough time to analyze it. Then it delayed the project for 90 days after the Environmental Protection Agency urged consideration of emissions from refineries in Texas and damage from potential oil spills.

Greens hope the State Department will embrace fresh delays because John Kerry, who took over from Hillary Clinton as Secretary of State in February, has long supported tackling global warming. “One would hope that Kerry … would be more than happy to spend more time talking about this,” said Damon Moglen, the climate director at Friends of the Earth.

RISK THEIR BODIES

An oil analyst said he expected the Obama administration to move toward a national interest decision this summer, but that some backers of the project fear more delays.

“What Keystone proponents fear is slow-walking the decision to death,” said Robert McNally, the head of the Rapidan Group, who worked in the White House as an adviser to former President George W. Bush. “Some fear the administration could decide to prolong the permit review process indefinitely, until TransCanada gave up.”

TransCanada, though, seems unfazed. Chief Executive Russ Girling said last week that the line could be built by late 2014 or early 2015 if a final decision comes by midsummer.

On Capitol Hill, Senator John Hoeven, Republican of North Dakota, is working with a majority in the 100-seat chamber to find a way to move legislation that would give Congress the power to decide the fate of Keystone. It is unclear whether they have the votes to do so, and President Barack Obama would likely veto the bill.

Still, environmentalists say the State Department review has served as a rallying cry. Daniel Kessler, a spokesman for 350.org, a group that has led demonstrations against the pipeline at the White House, said members will picket Obama and Kerry at home and abroad during the public comment period.

He said 350.org has been contacted by thousands of people willing to risk their bodies to stop the pipeline, and the group will train them in civil disobedience. Protests could take place in Washington, along the route of the proposed pipeline, and at TransCanada offices in Houston and near Boston, he said.

Once the State Department completes its review, greens could also file lawsuits to delay the national interest decision. Legal action could target the National Environmental Policy Act and clean water and endangered species laws.

Still, legal actions have not stopped construction of the southern leg of the Keystone pipeline. Building of that portion, which does not need a State Department permit because it does not cross the national border, has crossed the half-way point.

Adele Morris, an energy expert at the Brookings Institution, said Keystone opponents could hit the jackpot if they force long enough delays and U.S. oil prices, always unpredictable, fall far enough.

The current price of U.S. oil, at around $90 a barrel, is high enough to support oil sands development, which is more expensive than many other petroleum sources.

Investment activity could falter if prices fell sharply, but would pick up again when oil prices rose. “Keystone’s opponents are welcome to protest, but ultimately the price and demand for oil are more likely to impact the pace of oil sands development,” Morris said.