Meanwhile, the long-restive province of Quebec has worked for years to establish itself as a climate change-fighting leader within the Canadian federation. For Quebec, agreeing to link its cap-and-trade programs with California's in 2014 was a way for the province to meet its emissions targets and encourage other jurisdictions to join them.

But President Donald Trump has consistently chafed at — and sought to curtail — California's attempts to forge its own path. His administration sued to invalidate the climate pact in October, arguing that California was illegally seizing foreign policy powers that belong to the federal government.

The dispute fits nicely in Newsom’s wheelhouse; widely thought to have presidential visions, Newsom does all he can to highlight California’s resistance to the Trump administration.

And Legault has experience confronting national governments. A former Cabinet minister in a separatist Quebec government, he's now a popular leader of a nationalist party with ambitions to wrest powers — including on taxation and immigration — away from Ottawa for his province.

The lawsuit imperils California’s ability to use its enormous economy to claim a level of authority that makes it a rival power center to D.C. The U.S. Department of Justice was quite explicit about trying to neutralize that notion.

“California’s actions, as well as the actions of those acting in concert with it, have had the effect of enhancing the political power of that state vis-à-vis the United States,” the government’s complaint warned, adding that letting states “conduct their own foreign policy” is “anathema to our system of government and, if tolerated, would unlawfully enhance state power at the expense of the United States and undermine the United States’ ability to negotiate competitive international agreements.”

That legal battle forms the backdrop to Newsom and Legault’s meeting.

The Quebec premier and the governor will discuss strategies to defend the Quebec-California climate pact, Legault spokesperson Ewan Sauves said Tuesday. The leaders will also explore other ways they can collaborate to fight climate change.

Newsom was unavailable for an interview this week, but the administration has vowed to press ahead with its partnership with Quebec. When the Trump administration filed suit, adding to a long list of efforts to nullify or claw back California’s environmental regulations, Newsom accused the White House of “continuing its political vendetta against California” and seeking to stymie a program that's "a model for similar policies around the world."

Indeed, the partnership with Quebec is just one of dozens of interlocking agreements California has struck with other cities and countries — a form of climate diplomacy that former Gov. Jerry Brown championed as a counterweight or insurance policy against federal uncertainty.

California Air Resources Board chair Mary Nichols noted that the agreement with Quebec predated Trump’s election and predicted that “it will continue after Trump.” But she argued that federal pushback actually reinforces California’s position, and could embolden other states and provinces, by showing the necessity of forging new agreements despite “the failure of national governments to take action on the climate change crisis.”

“These agreements help to show others around the world that the president’s position, while certainly important, is not going to stop climate action,” Nichols told POLITICO. “It’s important for the cause and for our planet that we have allies in many places.”

Aimee Barnes, who was a senior Brown adviser on climate change, said those agreements are not designed to function in opposition to the federal government. They have “inherent value” in the all-hands-on-deck urgency of climate change, she said, noting that California was particularly aggressive in seeking new partnerships to bolster the Obama administration’s position going into landmark negotiations in Paris. But as those efforts continue, she said, Trump ends up looking isolated.

“There is momentum, so I think that whether the federal government wants to acknowledge it or not it exists, it’s irrefutable,” Barnes said. “I think we are seeing that at every level of government, except the White House.”

In Canada, “there’s a lot of regional variation” that makes it “really difficult for the federal government to do something” to reconcile Quebec’s openness with Alberta’s skepticism, said Mark Purdon, a professor at the Université du Québec à Montréal who has studied the California-Canada link.

“I think Quebec is learning from California, and people are learning from the California and Quebec experience of collaborating,” Purdon said.

But those agreements suffer from a critical weakness: They’re voluntary pacts, not legally binding international treaties. The same structure that makes them easier to create also makes them easier to exit when new politicians are elected — which is what happened when Ontario elected a conservative premier, Doug Ford, who dissolved a climate pact with California.

“What really matters is how you get others to follow, and cooperation between subnational governments like California and Quebec is often seen as a way to do that,” said Danny Cullenward, a Stanford economist who helps oversee California’s cap-and-trade program.

But that effort to generate some momentum doesn’t guarantee a durable result, Cullenward said, given that it’s unenforceable.

“It’s really just a political commitment that’s been written down,” Cullenward said. “It’s not a commitment that a party could take to a court in another jurisdiction and enforce.”

That fragility is “the big risk with all of these things,” Purdon acknowledged. But he said that risk has to be balanced against “the larger risk at the national level” of meaningful policies never emerging as the planet continues to warm.

“If you’re going to wait for the U.S. federal government to adopt a national plan, there’s going to be a waiting period to see if that goes ahead,” Purdon said. “That’s a bigger payoff if you get let’s see a national carbon tax or pricing system — a lot of people would see a lot of merit in that. But that just seems like a big risk.”

White reported from Sacramento and Blatchford reported from Ottawa.