A group of Democrats in Congress has written a letter to the State Department demanding an investigation into its handling of the controversial Keystone XL pipeline project.

The letter, from independent senator Bernie Sanders and 13 Democrats, was the second potential obstacle to TransCanada's hopes of beginning construction in the near future on the pipeline, after the state of Nebraska on Monday said it was exploring legal measures to re-route the pipeline away from an important aquifer.

The letter also flags up the political blowback to Obama of backing a project that has provoked so much opposition from liberal Democrats. The president was heckled by pipeline opponents at a visit to the University of Colorado.

Sanders, an independent from Vermont, joined 13 Democrats in seeking an investigation into the State Department's environmental review of the project.

Specifically, the letter asked the inspector general to look at allegations – first raised by environmental groups – of a possible conflict of interest, with a contractor for TransCanada hired to assess the environmental risks.

"Many serious concerns have been raised regarding conflicts of interest in the State Department's process for conducting its federally mandated review of this project," the letter to Obama said.

"On a decision of such consequences, with a project that could have a 50-year lifespan and that presents tremendous environmental and safety risks, we believe it is critical that the American people have confidence that all the facts have been presented in an objective and unbiased manner, and that the State Department and all private parties have fully complied with the letter and spirit of all federal laws and regulations."

Opponents of the pipeline said they were hopeful the IG would order an investigation. "A request from members of Congress carries serious weight," said Nick Berning of Friends of the Earth.

"This is a major blow to the State Department and its biased and flawed analysis of the pipeline's impacts."

Even before Wednesday's letter, there was speculation that the State Department would not meet its year-end deadline for rendering its decision on the project.

The prospects for a delay deepened further, after Wednesday's letter and moves by Nebraska to convene a special session of the legislature on 1 November to deal with the pipeline.

In calling the session, Nebraska's governor, David Heineman, said the session would explore legal means of moving the pipeline away from freshwater supplies.

Heineman said he opposes the pipeline project in his entirety but he said in a statement: "Nebraskans are expecting our best efforts to determine if alternatives exist. Therefore, I will be calling a special session of the Nebraska Legislature to have a thoughtful and thorough public discussion about alternative solutions that could impact the route of the pipeline in a legal and constitutional manner."

Opponents to the pipeline in Nebraska were surprised by the decision, and they warned that it was far from certain the legislature would be able to craft a bill.

"The big trick will now be having a bill to reroute the pipeline," said activist Jane Kleeb. "We hope that this will not just be political theatre for politicians to provide cover for themselves."

She said activists were waiting to see if Heineman comes up with his own proposals, or would support efforts by Annette Dubas, a state senator, to craft a bill re-routing the pipeline.

Dubas's latest effort would give the state public service commission authority to approve pipeline routes.

That kind of alteration to the pipeline's proposed course could hold up the project for months, possibly forcing TransCanada to reconsider the entire project.

But gathering those votes might prove difficult. Legislation for emergency action requires a two-thirds vote of the legislature.