WASHINGTON — President Barack Obama is trying to negotiate a legacy-making climate change pact this coming week in Paris with one hand tied behind his back. Congress can’t even agree whether global warming is real.

Scientists point to the global agreement, years in the making, as the last, best hope for averting the worst effects of global warming. Obama has spent months prodding other countries to make ambitious carbon-cutting pledges to the agreement, which he hopes will become the framework for countries to tackle the climate issue long beyond the end of his presidency in early 2017.

But Republicans have tried to undermine the president by sowing uncertainty about whether the U.S. will make good on its promises. Senate Majority Leader Mitch McConnell, R-Ky., and other GOP leaders have warned other countries not to trust any deal Obama may strike; other GOP allies are working to nullify Obama’s emissions-cutting steps at home.

“America is extremely divided, and there doesn’t seem to be any prospect that’s going to change at least in the next year or two,” Gov. Jerry Brown, D-Calif., who is attending the talks, said in an interview. “America’s leadership is not as great as it should be given the recalcitrance and the continuing obstructionism of the opposition party.”

About 150 heads of state are set to join Obama for talks on Monday and Tuesday as the deal nears the finish line. The goal is to secure worldwide cuts to emissions of heat-trapping gases to limit the rise of global temperatures to about another 2 degrees from now.