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President Barack Obama's determined efforts to combat global warming face their biggest trial yet as Republicans take full control of Congress this week. The GOP vows to move fast and forcefully to roll back his environmental rules and force his hand on energy development.

The GOP's first order of business: the Keystone XL pipeline. The Republican-led House has repeatedly passed legislation to approve the pipeline, which would carry tar sands oil from Canada deep into the United States. The bills died in the Senate when Democrats were in control, but that will change Wednesday when a Republican-led Senate committee holds a Keystone hearing.

"The president is going to see the Keystone XL pipeline on his desk and it's going to be a bellwether decision by the president whether to go with jobs and the economy," Sen. John Barrasso, R-Wyo., said Sunday.

Success for Republicans on the climate front would jeopardize a key component of Obama's legacy. And the ramifications would likely ricochet far beyond the United States.

Later this year, nations are supposed to sign a major global climate treaty in Paris. Aggressive action by the U.S. under Obama has upped the pressure on other governments to get serious about climate change, too. But if Obama can't make good on his commitments at home, it's unclear whether poorer nations will still feel compelled to act.

"The American government has been responsible for sending very strong political and economic signals with what they have announced so far," former Mexican President Felipe Calderon, now a global climate leader, said in an Associated Press interview. "I know that there is a risk that those will be overcome by the new political reality in the U.S."

Obama has made clear he will use his veto power if Republicans succeed in getting hostile bills to his desk ? especially on climate change. "I'm going to defend gains that we've made on environment and clean air and clean water," he has said.

And Sen. Brian Schatz, D-Hawaii, says the Republicans aren't likely to overturn his veto. That would require a number of Democrats to vote against the president.

"There's reason to be concerned, but I don't think there's reason to be panicked," Schatz said.

By design, Obama's biggest steps on climate rely on existing laws and don't explicitly require Congress to act. But Republicans can try to undercut them before they take effect. Republicans argue that Obama's coal plant emissions limits, for example, would devastate local economies and hamper job-creation.

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Big Threat for Obama's Climate Efforts From GOP-Run Congress