CHICAGO—Former President Barack Obama popped in to Mayor Rahm Emanuel’s mayoral climate summit on Tuesday, saying it can be challenging to address climate change “in an unusual time” but never mentioning U.S. President Donald Trump by name.

The former president’s Chicago appearance also gave a boost to his former chief of staff as Emanuel ramps up to run for a third term as mayor.

Obama was in town for a private speech later in the day and talked to mayors in attendance at the downtown climate event for about 15 minutes. Though Trump’s pulling the U.S. out of the Paris climate agreement prompted mayors to take steps to limit greenhouse emissions themselves, Obama stepped around criticizing Trump directly.

Instead, the former president tried to show a path toward fighting climate change without Trump’s cooperation.

“Obviously we’re in an unusual time when the United States is now the only nation on earth that does not belong to the Paris agreement,” Obama said. “And that’s a difficult position to defend. But the good news is that the Paris agreement was never going to solve the climate crisis on its own. It was going to be up to all of us.”

Reciting a litany of recent extreme weather events he said are evidence of climate change, Obama also mentioned the aftermath of Hurricane Maria’s devastation of Puerto Rico, a recovery effort for which the Trump administration has drawn criticism.

“A conveyor belt of some of the strongest hurricanes on record this summer smashed into Houston, and Florida and Puerto Rico, and more than two months later they are still struggling to recover,” he said.

And Obama called on American voters to “make it a prerequisite for the vote for anybody for elected office that they’re paying attention” to global warning. His remarks Tuesday are in keeping with Obama’s general policy of not engaging directly with Trump since the new administration took over in Washington, D.C.

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While Obama stuck to talk about the environment, Emanuel placed Trump front and center at the event by pointing out he has opted the U.S. out of the Paris climate accords.

The preamble to the Chicago Climate Charter the participating mayors signed on Tuesday read: “President Trump’s decision to withdraw the United States from the Paris Agreement spurred over 380 cities in the U.S., numerous cities in other parts of North America, and countless organizations worldwide to commit themselves to the Agreement.”

And Emanuel was champing at the bit to talk Trump at a news conference Tuesday. When a reporter asked Canadian mayors what message they had for Trump, Emanuel interjected: “Can I take that question too?”

“We can’t afford, as leaders of our respective cities, the absence of leadership and the wrongheaded policy by President Trump. Can’t afford it,” the mayor said.

“Would I prefer a national government and a national leader like President Obama, who led the country the right way, and then have that partner? Every one of us would,” he added. “Can I afford, even if we don’t have a partner, to stand still and just blame him? No, that’s not leadership.”

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Obama was already in Chicago Tuesday for a long-planned speech to the venerable Economic Club of Chicago that is closed to the public. He opened his remarks at the climate conference by embracing Emanuel onstage, then praising him. “This is my hometown, so I hope Rahm has been as good of a host as he was a chief of staff, and as good as he has been a mayor,” Obama said. “Thank you, Rahm, for your outstanding leadership.”

His support has been important to Emanuel’s political fortunes in the past.

As president, Obama held a high-profile send-off in the White House for Emanuel as he left the administration in 2010 to come run for mayor for the first time. Video clips of Obama praising Emanuel at the event later turned up in an Emanuel campaign ad.

Obama came to Chicago to support Emanuel again during the mayor’s 2015 re-election campaign, and a clip from that event formed the basis of a TV commercial Emanuel ran in the days before the vote.

Emanuel went all in on the pomp and circumstance at the Tuesday event even before Obama arrived. Participating mayors from suburban Hanover Park and Franklin Park, and those from Paris, Mexico City, Honolulu and other U.S. and world cities, were invited up on stage to add their signatures to the charter as their colleagues applauded.

While the Chicago charter calls on participating cities to set their own standards to reduce emissions, it’s not clear whether such local action will be able to offset federal shortcomings or how local governments will be held to their commitments.

The climate meeting was just the latest opportunity Emanuel has taken to dog Trump. He hosted a water cleanliness meeting with mayors from around the world in March, an occasion he used to call on the president not to cut the Environmental Protection Agency budget for the Great Lakes.

“To endanger the environmental progress made over 30, 40 years, rolling those dollars back, I think they’ll face a fight, but that fight will come at the expense of both our recreation and more importantly our public health and environmental quality and sustainability of Lake Michigan and all the other Great Lakes,” he said then.

That news conference also was noteworthy for Emanuel recalling that in his youth “dead fish just rolled in” on Chicago beaches before cleanup efforts.

It’s all part of Emanuel’s broader effort to stand at the forefront of mayors mounting high-profile challenges to the Trump administration. Emanuel has ceaselessly hammered Trump as against immigrants while promoting Chicago at every opportunity as a city that will welcome and protect them. In one case, he sued the Trump Justice Department in federal court over immigration rules the administration wanted to tie to the issuance of public safety grants.

Asked Tuesday at the news conference about the climate charter, Emanuel immediately pivoted to point out the lack of leadership from Trump “across a waterfront of issues,” and brought up the immigration lawsuit.

The anti-Trump stances also fit into the mayor’s long-standing position that we are living in “the century of the city.” It’s a slogan he sometimes applies when discussing the ways municipal governments can continue to get things done for the betterment of residents in spite of gridlock at the federal and state levels.

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