ASHINGTON ? President Obama vetoed a bill Tuesday that would have approved the Keystone XL pipeline, making good on a threat to reject a proposal embraced by Republicans as a jobs measure but opposed by environmentalists as contributing to climate change.

"The presidential power to veto legislation is one I take seriously," Obama said in his veto message. "But I also take seriously my responsibility to the American people. And because this act of Congress conflicts with established executive branch procedures and cuts short thorough consideration of issues that could bear on our national interest ? including our security, safety, and environment ? it has earned my veto."

It was only the third veto of his presidency, but likely to be the first in a series of vetoes as he parries a Republican-controlled Congress in the last two years of his presidency. The White House has already issued 13 formal veto threats so far this year ? the most ever at this point in a new Congress since President Reagan first started issuing written veto threats in 1985.

Obama vetoed the Keystone bill shortly after it arrived at the White House from the Senate, Senate Majority Leader Mitch McConnell had held up its transmittal so that Congress could be in session when it went to the president.

White House Press Secretary Josh Earnest said the president vetoed the bill almost as soon as it arrived at the White House, "without any drama or fanfare or delay."

An override of the Keystone veto is unlikely. The bill passed the House 270-152 and the Senate 62-36, margins well short of the two-thirds majorities needed to override. McConnell said a veto override vote will be scheduled by March 3.

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The 1,179-mile pipeline would connect tar sands cruse oil in Alberta Canada with an existing pipeline in Nebraska, allowing energy company TransCanada to pump 830,000 barrels a day to refineries in the Midwest and the Gulf Coast.

Obama's veto doesn't mean the end of the Keystone debate. Obama could still approve the project on his own authority, although he has suggested that its environmental impacts would outweigh any economic benefits.

In the meantime, the State Department continues to review the proposal to determine whether it's in the national interest. TransCanada first applied for permits to build the pipeline in 2008, and there's no indication of when the review will be finished. "It's an ongoing process that doesn't have a deadline," State Department spokeswoman Jen Psaki said this month.

TransCanada said in a statement it remains committed to the project and is working with the State Department to resolve environmental concerns.

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Also, congressional Republicans have said they could attach a Keystone provision to future bills, in an effort to reach some kind of compromise that could either earn the president's signature or get enough votes to override his veto.

"The allure of appeasing environmental extremists may be too powerful for the president to ignore," wrote McConnell and House Speaker John Boehner in an op-ed in USA TODAY. "But the president is sadly mistaken if he thinks vetoing this bill will end this fight. Far from it. We are just getting started."