WASHINGTON — If Republicans push through Congress a measure approving the long-stalled Keystone XL pipeline, Democrats would have the votes to uphold a presidential veto, a top Senate Democrat said Sunday.

Republican leaders say that when lawmakers return to Congress this week, a bill approving Keystone, which has support of most Republicans and some Democrats, will be an early priority.

But Sen. Charles E. Schumer of New York, third-ranking Democrat in the Senate, said President Obama should not sign a bill approving the pipeline.

Obama should reject the pipeline even if Congress approves amendments making it more palatable to opponents, such as a requirement that all the oil transported by the pipeline stay in the United States, Schumer said.

“You know our Republican colleagues are doing what they always do: They’re appeasing a few special interests, in this case oil companies and pipeline companies, and not really doing what’s good for the average middle class family in terms of creating jobs,” he said on CBS’s “Face the Nation.”

The planned 1,179-mile pipeline, intended to transport tar sands oil from Alberta, Canada, to refineries in the Gulf of Mexico, has become a symbolic issue for both sides: Environmentalists say it will worsen global warming, while backers say the project will provide jobs.

Obama has not said whether he would veto Keystone legislation.

But his public statements about the proposed pipeline have become increasingly skeptical over the past year.

The president has noted that the project would have little positive economic impact – the pipeline won’t require many workers to operate once it’s built – and has stressed the potential environmental problems.

Overriding a presidential veto requires a two-thirds vote in both houses of Congress. Republicans have majorities in both the House and Senate, but their numbers fall considerably short of that mark.

Polls show that overall the public supports building the pipeline by nearly 2 to 1, but support has declined over the last year.

The decline in support has been particularly sharp among Democrats, who are now evenly divided on the issue, according to a recent Pew Research Center survey.