Nebraska Gov. Dave Heineman urged President Barack Obama and U.S. Secretary of State Hillary Clinton on Wednesday to deny a federal permit for a pipeline that would carry Canadian oil over an aquifer that supplies drinking and irrigation water to parts of several states.

Heineman said he supports pipeline projects but opposes the proposed TransCanada Keystone XL route that would cross the vast Ogallala aquifer.

In a letter to Obama and Clinton, the Republican governor said he was concerned about the potential threat to the crucial water source for Nebraska's farmers and ranchers. The aquifer also supplies drinking water to about 2 million people in Nebraska and seven other states.

"This resource is the lifeblood of Nebraska's agricultural economy," Heineman said in the letter.

"Cash receipts from farm markets contribute over $17 billion to Nebraska's economy annually. I am concerned that the proposed pipeline will have potentially detrimental effects on this valuable natural resource and Nebraska's economy."

Heineman said he disagrees with a part of a final environmental impact statement released by the State Department last week, which said a petroleum spill in the aquifer would affect a limited area.

The pipeline project is designed to carry oil from Canada across Montana, South Dakota, Nebraska, Kansas and Oklahoma on its way to refineries on Texas' Gulf Coast.

Calgary-based TransCanada submitted its Keystone XL project in late 2008 to the State Department, which has authority over the pipeline because it crosses an international boundary.

Heineman released the letter in the midst of ongoing pipeline protests in Washington, D.C. On Tuesday, actress Daryl Hannah was arrested along with other environmental protesters who ignored orders to move away from the White House.

Environmental groups fear the pipeline could foul underground and surface water supplies, increase air pollution around refineries and harm wildlife. They have criticized what they consider inadequate pipeline safety and emergency spill responses.

Supporters say the environmental concerns are overblown, and argue that the pipeline will create jobs and reduce the nation's dependence on Middle East oil.

TransCanada spokesman Terry Cunha said the company disagreed with the governor, but was open to any questions he might have about the route.

Cunha said moving the route would disrupt even more land, including wetlands and more developed areas. He said the plan has won support from several other state executives, including Texas Gov. Rick Perry and Montana Gov. Brian Schweitzer.

"We believe we're building the safest pipeline in North America," Cunha said. "As we've tried to articulate during the three-year review process, we've been very open and transparent" with more than 90 public hearings in the affected states.

The pipeline also has won support from the U.S. Chamber of Commerce, which launched a campaign this week to highlight the estimated 20,000 jobs the line would create during the two-year construction period, as well as its potential national security benefits.

Pipeline critics praised Wednesday's letter as a step forward, but said the governor still needs to call for a special legislative session or sign an executive order to address pipeline concerns.

Jane Kleeb, the executive director of the left-leaning group Bold Nebraska, said she was surprised by the announcement and hailed it as sign of growing opposition to the pipeline.

But Kleeb said the governor also is trying to divert attention from the state's power to determine the pipeline route.

"We obviously are very happy that he's decided to side with Nebraska farmers and ranchers, rather than a foreign oil company," Kleeb said. "But there's an important distinction that needs to be made here. What he's leaving out is that the route of the pipeline is up to him."

Congressional lawmakers from both parties have written to the State Department arguing for and against the pipeline, but some of the strongest opposition has come from Nebraska, where the state's two U.S. senators have raised sharp questions.

Democratic Sen. Ben Nelson said Nebraska has had the opportunity for more than a year to weigh in on the Keystone proposal.

"If the governor doesn't like the route, he has the power to change it," Nelson said Wednesday. He noted Democratic state Sen. Ken Haar and others "are working to change the route, and I'd encourage the governor to work with them in exercising the state's rights."

Haar has called for a special session and said he is exploring options to prompt state action. He said the governor's letter "accurately reflects the opinion of most Nebraskans" who support the pipeline but oppose the proposed route.

U.S. Rep. Jeff Fortenberry, a Nebraska Republican, said he is urging a comprehensive federal review to address the line's potential impact on the aquifer and the ecologically diverse Sandhills.

"An alternative route should be considered," he said.