WASHINGTON — President 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 Republican leaders have warned other countries not to trust any deal Obama may strike; other Republican allies are working to nullify Obama’s emissions-cutting steps at home.

“Congress and more than half of the states have already made clear that he won’t be speaking for us,” according to a McConnell opinion column posted on The Washington Post’s website. He said it would be “irresponsible for an outgoing president to purport to sign the American people up” for a new climate agreement.

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.

With little room for error, leaders have tried to avoid the pitfalls that undercut global climate negotiations in the past – specifically, those in Kyoto, Japan, in the early 1990s and in Denmark during Obama’s first term.

FIRST TO INVOLVE ALL COUNTRIES

The deal in Kyoto – which the U.S. never ratified – spared developing countries such as China and India from mandatory emissions cuts, causing resentment in the U.S. and other industrialized countries. The Paris agreement would be the first to involve all countries.

The concept behind a Paris pact is that the 170 or so nations already have filed their plans. They would then promise to fulfill their commitments in a separate arrangement to avoid the need for ratification by the Republican-run Senate. That dual-level agreement could be considered part of a 1992 treaty already approved by the Senate, said Nigel Purvis, an environmental negotiator in the Clinton and George W. Bush administrations.

But it’s not just about whether or not to ratify.

In the United States, the talks are entangled in the debate about whether humans really are contributing to climate change, and what, if anything, policymakers should do about it. Almost all Republicans, along with some Democrats, oppose the steps Obama has taken to curb greenhouse gas emissions

Half the states are suing the administration to try to block Obama’s unprecedented regulations to cut power plant emissions by roughly one-third by 2030. If their lawsuit succeeds, Obama would be hard-pressed to deliver the 26 percent to 28 percent cut in overall U.S. emissions by 2030 that he has promised as America’s contribution.

Opponents also are trying to gut the power plant rules through a rarely used legislative maneuver that already has passed the Senate. A House vote is expected while international negotiators are in Paris.

Senate Republicans are working to block Obama’s request for the first installment of a $3 billion pledge to a U.N. fund to help countries adapt to climate change, a priority for poorer countries.