In the simple world of civics class, the president gets to make treaties and they’re binding on the United States when two out of three senators say so.

In today’s complex political world, that’s almost never how it plays out. Beyond baked-in partisanship and steep distrust of whoever occupies the White House lies this obstacle to Senate ratification of any international agreement: The protection of American sovereignty is among the most basic conservative objectives.

That helps explain why 2010 was the last time two-thirds of senators lined up to approve a treaty, and why the Obama administration now wants to make sure no foreign policy agreements get labeled with “the T word.” This year’s Iran nuclear agreement would have had zero chance had it been written as a treaty; the same would be true for any document produced at the environmental summit in Paris. (The 54 Republican votes are a score more than what’s necessary to guarantee Senate rejection of any treaty.)

All those realities leave Barack Obama to rely on his own robust view of a president’s foreign policymaking powers to advance his climate agenda.

This makes the most confrontational GOP senators almost as angry as the president’s environmental views themselves. It also makes much of the rest of the world wary of any American commitment to combating climate change.

Many of the other 190 nations in the Paris talks want a legally binding and globally enforceable treaty document instead of promises (especially by a president a year from the time he exits office) about how the United Sates will do its part to combat global climate change.

While the administration says there will be “legal force” behind whatever agreement it makes, “There’s no consensus on what those words really mean, or how that will work, or who will decide which parts of any agreement are legally binding and which are not,” says Frank E. Loy, the chief climate negotiator in the final three years of the Clinton administration.

That vagueness can be a virtue, he said in an interview, if the end result is simultaneously satisfying to a skeptical world and irreversible by an antagonistic Congress.

But the same situation puts the president “in a terrible bind,” two senior foreign policy officials in the George W. Bush administration, John Bolton and John Loo, wrote on Dec. 1 in The Los Angeles Times. “Obama can achieve his climate change legacy only through delicate negotiations with Congress. His poor relations with the House and Senate, especially on foreign policy, appear to render success unlikely. Obama may rely on his unilateral authority to join a world climate pact, but without Congress his most important promises will be empty ones whose fate will be left to his successor.”

That theory looks to face another test at the Capitol even before the ink on any agreement dries in France.

Obama has committed $3 billion to a United Nations fund for helping developing nations pay for lower-carbon energy generation and minimizing consequences of climate change. The robustness of this Green Climate Fund, with almost a third of the money coming from the United States, is essential to persuading those countries to commit to reductions in greenhouse gas emissions while sustaining economic growth, which usually means reliance on cheaper fossil fuels.

Some powerful Republicans are hoping to use the omnibus appropriations package to prevent the State Department from delivering the first $500 million installment of Obama’s commitment — unless he subjects any Paris agreement to Senate ratification.

The president “is so desperate for a climate deal in Paris that he will do anything he can to get one. This includes undermining U.S. sovereignty and misusing American taxpayer dollars to grease the wheels,” John Barrasso of Wyoming, the Senate GOP Policy Committee chairman, said on Dec. 4.

His side does not appear to have the votes to get what it wants; only 37 senators and 110 House members have signed on letters supporting the idea. And the White House has signaled such a rider is on the list of GOP proposals that would be veto bait on the year-ending spending bill.

Another Republican move to undermine Obama’s negotiating position is even more certain to come to naught. The House recently cleared a pair of measures, already passed by the Senate, to block EPA rules for cutting pollution at both future and exiting coal-fired power plants. (Reducing gas-trapping carbon emissions by at least 26 percent in the next two decades is the main promise Obama’s team took to the summit.)

The majorities were several dozen short of the votes required to override Obama’s certain vetoes — but still big enough to remind other nations how deeply divided the U.S. government is on climate change. Diplomats abroad already know full well how resistant Senate Republicans are to making promises about future U.S. behavior

The last treaty ratification was in December 2010, and only a third of the Republicans voted for a nuclear arms reduction deal with Russia then, even though it had been endorsed by all five living former GOP secretaries of State.

December 2012 was the last time a treaty was put to a roll call, and it failed by six votes. Despite their former leader, Bob Dole, working the floor in his wheelchair to round up support, only eight Republicans backed a pact committing the world’s nations to protecting the rights of their disabled populations.

The governing international treaty to combat global warming, reached in Kyoto, Japan, in December 1997, was never even submitted to the Senate. That’s because, five months in advance, senators voted 95-0 for a resolution declaring that such a deal must require India, China and other developing nations to cut their emissions by specific levels. The final deal didn’t do so, but would have required a 7 percent cut from the United States over 12 years.

“What they are negotiating in Paris should put us on a better road than Kyoto,” said Loy, who became top climate negotiator just after that dead-on-arrival moment in the Senate. “That’s because it takes account of the reality that what each country can do on the environment is related to its unique domestic political situation.”

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