Washington ? President Barack Obama and Congress are starting the election year locked in a tussle over a proposed 1,700-mile oil pipeline from Canada to Texas that will force the White House to make a politically risky choice between two key Democratic constituencies.

Some unions say the Keystone XL pipeline would create thousands of jobs. Environmentalists fear it could lead to an oil spill disaster.

A law Obama signed just before Christmas that temporarily extended the payroll tax cut included a Republican-written provision compelling him to make a speedy decision on whether to build the pipeline. The administration is warning it would rather say no than rush a decision in an election year.

It’s a dicey proposition for Obama, who enjoyed strong support from both organized labor and environmentalists in his winning 2008 campaign for the White House.

Environmental advocates, already disappointed with his failure to achieve climate change legislation and the administration’s decision to delay new smog standards, have made it clear that approval of the pipeline would dampen their enthusiasm for Obama in the upcoming November election.

Some liberal donors even threatened to cut off funds to Obama’s re-election campaign to protest the project, which opponents say would transport “dirty oil” that requires huge amounts of energy to extract.

If he rejects the pipeline, Obama risks losing support from organized labor, a key part of the Democratic base, for thwarting thousands of jobs.

Obama appeared to have skirted what some dubbed the “Keystone conundrum” in November when the State Department announced it was postponing a decision on the pipeline until after this year’s election. Officials said they needed extra time to study routes that avoid an environmentally sensitive area of Nebraska that supplies water to eight states.

The affected area stretches just 65 miles through the Sandhills region of northern Nebraska, but the concerns were serious enough that the state’s governor and senators opposed the project until the pipeline was moved.

Republican Gov. Dave Heineman, who opposed the initial route, says he supports efforts to accelerate the project, noting that provisions in the payroll tax bill allow the project developer to find a new route avoiding the Sandhills.

The new route would have to be approved by Nebraska environmental officials and the State Department, which has authority because the pipeline would cross an international border.