President Barack Obama vetoed the Republicans’ Keystone XL pipeline bill Tuesday, rejecting Congress’ attempt to take the project’s fate out of his hands — and leaving the GOP on track for an override vote that will most likely fail.

Obama’s veto message was delivered within hours of the legislation arriving on his desk, the latest step in six years of pushing and pulling over a Canada-to-Texas oil pipeline that has become a symbol in the debate over jobs versus the environment. Even before Obama whipped out his veto pen, Senate Majority Leader Mitch McConnell vowed to hold an override vote, which the Republican leader’s office said would occur no later than March 3.

That gives the GOP a small window to search for the Democratic votes they still need in order to push the bill past Obama. About 20 more Democratic votes in the House and four in the Senate are required to enact the bill, judging from the votes the Keystone bill got when Congress passed it last month.

In a veto message to Congress, Obama rejected the Keystone legislation as an attempt to “circumvent longstanding and proven processes for determining whether or not building and operating a cross-border pipeline serves the national interest.” The bill would have declared the pipeline approved, ending the administration’s review of the project.

“The Presidential power to veto legislation is one I take seriously,” said Obama, who was vetoing a bill for only the third time in his presidency. “But I also take seriously my responsibility to the American people.”

Republicans, and some pro-fossil-fuel Democrats, argue that the “proven” process has dragged on far too long for Keystone.

But green groups cheered the veto, which they see as putting the White House well on track to ultimately rejecting Keystone. Obama has no binding deadline to make the final decision on the pipeline’s permit, but Secretary of State John Kerry could make his own recommendation on the issue as soon as this week.

“President Obama said he’d veto this attack on his executive authority, and he kept his word,” Sierra Club Executive Director Michael Brune said in a statement that criticized House and Senate leaders for spending so much time on a doomed bill. “That’s what he said he’d do from the start, but Republicans in Congress continued to waste everyone’s time with a bill destined to go nowhere, just to satisfy the agenda of their big oil allies.”

Republicans expressed disappointment but not surprise.

“I’m not shocked obviously,” Senate Energy and Natural Resources Chairwoman Lisa Murkowski (R-Alaska) said. “I’m disappointed because I think the president is missing an opportunity when it comes to jobs and North American energy independence.”

House Speaker John Boehner labeled the veto “a national embarrassment,” adding that “we are not going to give up in our efforts to get this pipeline built — not even close.”

Republicans expected the veto, but they nonetheless seized the opportunity to accuse Obama of bowing to an extremist green agenda that would rob the U.S. of thousands of construction jobs and other economic benefits. The Senate GOP Conference sent out a statement Tuesday afternoon lambasting Obama as the “Obstructor in Chief.”

Meanwhile, the Canadian government put on a brave face, predicting that Keystone will eventually be built.

“It is not a question of if this project will be approved; it is a matter of when,” Canadian Natural Resources Minister Greg Rickford said in a statement. He added, “This is not a debate between Canada and the U.S.; it’s a debate between the President and the American people, who are supportive of the project.”

Prime Minister Stephen Harper has been an ardent supporter of the $8 billion pipeline, even personally lobbying Obama to approve it. The Canadian government has also spent $24 million on a pro-oil advocacy campaign that included ads blanketing D.C. Metro stations near the White House.

The pipeline’s developer, TransCanada, said in a statement that it remains “remains fully committed to Keystone XL” and maintained again that the project meets Obama’s previously stated demand that it not significantly worsen climate change. CEO Russ Girling also pointed to statistics showing that pipelines are a safer way to transport oil than rail, barge or truck — not mentioning last week’s fiery derailment of an oil train in West Virginia.

“While groups opposed to the safe and sustainable transportation of products such as oil by pipeline may try to claim a symbolic victory, it’s important to focus on what is at stake,” Girling said in a statement.

But Sen. Ed Markey (D-Mass.) called on Obama to follow the veto by rejecting Keystone once and for all.

“We should not help some of the dirtiest oil in the world to be funneled through our country like a straw, just so much of it can be exported to foreign nations,” Markey said. “That is a bad deal for our country, and a dangerous proposition for our climate.”