The Obama administration once again has punted on a final decision for the Keystone XL pipeline, announcing ahead of the holiday weekend it is extending a key review period indefinitely -- a move that could push off a determination until after the midterm elections.

Republicans, as well as red-state Democrats who want the proposed Canada-to-Texas pipeline approved, slammed the administration for the delay. Democrats even threatened to find ways to go around the president to get the project approved.

"It's absolutely ridiculous that this well over five year long process is continuing for an undetermined amount of time," Sen. Heidi Heitkamp, D-N.D., said in a statement.

Republican Nebraska Rep. Lee Terry called the decision "shameful," noting that another spring construction season will come and go without the project.

The administration had been in the middle of a 90-day review period for federal agencies assessing an environmental study from the State Department.

But the State Department said Friday it is giving agencies "additional time" to weigh in, in part because of ongoing litigation before the Nebraska Supreme Court which could affect the pipeline's route. If the route changes, officials made clear the State Department reserves the right to conduct another environmental impact study to include more public comments, which could delay the process more.

Further, the department said officials need to go over the "unprecedented number" of new public comments -- roughly 2.5 million of them -- received during a separate comment period that ended in early March.

"The Permit process will conclude once factors that have a significant impact on determining the national interest of the proposed project have been evaluated and appropriately reflected in the decision documents," the department said.

Keystone supporters in Congress were furious with the decision. Just days earlier, 11 Democratic senators had written to President Obama urging him to make a final decision by the end of May, complaining that the process "has been exhaustive in its time, breadth and scope."

With the extension, the administration effectively has turned down that request. One of the letter's signatories, Sen. Mary Landrieu, D-La., who is in a tough re-election fight this year, said the decision amounts to an "indefinite delay" of the project.

"This decision is irresponsible, unnecessary and unacceptable," she said. "By making it clear that they will not move the process forward until there is a resolution in a lawsuit in Nebraska, the administration is sending a signal that the small minority who oppose the pipeline can tie up the process in court forever. There are 42,000 jobs, $20 billion in economic activity and North America's energy security at stake."

Landrieu, as chairwoman of the Senate Energy and Natural Resources Committee, also threatened to "take decisive action to get this pipeline permit approved."

This could put pressure on Senate Majority Leader Harry Reid to allow a vote on legislation to either force the president to approve the project or make a decision by a certain date.

There is a wide bipartisan support in the Senate. Last year, the chamber voted 62-37 on a nonbinding amendment that called for the pipeline's approval.

A congressional source told Fox News that at this point, the administration is not giving a hard deadline, and the process appears to be open-ended.

The administration has faced pressure from both sides of the debate on the pipeline. Republicans largely are united in support of the project, but Democrats are sharply divided. Moderate Democrats, as well as labor unions, are pressing the State Department to give the thumbs-up, calling the project a jobs engine and a way to boost energy security. But environmental interests, and lawmakers allied with them, are strongly opposed -- citing its alleged impact on climate change and possible health risks.

One such environmentalist, California billionaire Tom Steyer, has been vowing to back vulnerable Democrats with big money if they oppose the pipeline. He called the latest announcement "good news on Good Friday for those who oppose Keystone as not being in our nation's best interest."

The State Department has jurisdiction because the Canada-to-Texas pipeline crosses the U.S.-Canada border. The drawn-out process has frustrated America's allies in Canada. Canadian Ambassador to the U.S. Gary Doer told Fox News after the announcement Friday that every day the project is delayed is a lost opportunity to put people to work.

He also said the oil is coming "whether it be by rail or truck."

In pointing to the Nebraska court case, the State Department was referring to a state judge recently overturning a law allowing the current path through the state.

But Sen. John Hoeven, R-N.D., charged that the strategy here "is clearly defeat through delay."

Fox News' Mike Emanuel, Doug McKelway, Kara Rowland, Justin Fishel and Chad Pergram contributed to this report.