CHARLOTTE, N.C. — In 2008, Al Gore strode onto the stage at Denver's Invesco Field to a hero's welcome, throwing his support behind Barack Obama to take on the "global climate crisis."
When Obama takes the stage this week, Gore will be nowhere in sight.
He isn't coming to the Democratic National Convention but is spending the week in New York City, anchoring coverage of the event for his network Current TV.
Gore's evolution over the past four years — from a central figure in the Democratic Party to a no-show at its biggest event — matches what has happened to the issue of climate change itself, which moved to the sidelines alongside its chief crusader, environmentalists and some Democrats say.
It's not like Gore hasn't noticed — and his frustration with Obama has been on display. He's leveled criticism at Obama for abandoning the push for a climate change bill. He accused him of failing to use the bully pulpit to spread the word about the dangers of rising global temperatures. And he faulted Obama for putting off tough new smog regulations.
On the other hand, Gore has also offered some defense of Obama's record and says that "I would fear for the future of our environmental policy" if Mitt Romney wins the election.
People who know Gore say this is the role where he feels he can make a difference now — critic and outsider, more activist than politician.
"He is not someone who is inside the system right now," said Chris Lehane, a political consultant who worked in the Clinton White House and later on Gore's 2000 presidential campaign. "He is someone that recognizes that ... you need someone who is a critical voice and picks and chooses when he needs to push.
"He's obviously a Democrat and will always be a Democrat," Lehane added. "But he's spent an awful lot of time engaging in activities that are above and beyond partisan politics."
While many Democrats in Charlotte were reluctant to talk about Gore for attribution, some said Gore's diminished profile — and the environmental movement's more broadly — is rooted in the mood of a country preoccupied with questions about the economy and jobs.
"Al Gore has proven incapable of developing policies that have broad political appeal, and the party is looking to others," said Paul Bledsoe, who was communications director for the White House Climate Change Task Force under President Bill Clinton and is now a senior adviser at the Bipartisan Policy Center.
Carl Pope, who served as the executive director of the Sierra Club for almost 20 years, also sees the shift in Gore's role, especially as prospects for Congress to act on climate change have dimmed.
"Like all of us, I think, Gore is in the process of testing and figuring out what the best pathway forward is," Pope said. "He sees that the public arena is at the moment more critical than the political arena."
It's unclear whether Gore wanted a role this year at the convention. He announced plans to anchor Current TV's convention coverage from New York before organizers finished sending their speaker invitations.
Aside from any differences with the administration on policy, some events in the former vice president's personal life may have made him less than ideal for prime time exposure in Charlotte. They include his separation from his wife, Tipper, and the unproven assault allegations that authorities in Portland, Ore., cleared him of two years ago.
But his outside role is a contrast to the prominent speaking slot Obama has granted to Clinton, who serves to remind voters of the prosperous economy that reigned during the last Democratic administration.
As for the relative quiet on climate change, Sen. Sheldon Whitehouse faults his own party, saying Democrats have allowed the GOP to intimidate them from talking up the issue.
"I think the Republicans do a very good propaganda job of poisoning certain phrases. And then, rather than fight, we walk a step back, and we use a different phrase," said the Rhode Island Democrat, who didn't address questions about Gore specifically. "Once they poison that phrase, we'll walk back again. So, we say: 'We won't talk about climate change anymore. Now we're going to talk about clean energy.' "
Four years ago, Gore had a starring role on the final night in the 75,000-capacity stadium in Denver, where he was one of the last few speakers chosen to stir up the crowd before Obama's acceptance speech.
Gore himself was coming off a year of triumphs, including his Nobel Peace Prize and the Oscar awarded to his global-warming documentary An Inconvenient Truth. And he assured voters Obama would take action on the issue that George W. Bush had neglected.
"We are facing a planetary emergency, which, if not solved, would exceed anything we've ever experienced in the history of humankind," Gore said. Obama, he promised, would offer "solutions for the climate crisis."
Obama himself had stoked such hopes earlier in the 2008 campaign, famously promising at one point that "this was the moment when the rise of the oceans began to slow and our planet began to heal."
But this time around, climate change has all but disappeared from Obama's campaign rhetoric, although his recent speeches have included a line that "denying climate change won't make it stop" and climate remains part of the DNC platform.
Some greens remain frustrated.
"It's a little puzzling that, as America sets new records for heat, the Obama campaign has been so silent about climate," said Bill McKibben, president and co-founder of the climate activist group 350.org, whose mass White House sit-ins last year against the Keystone XL pipeline were one of the most successful environmental protests in recent memory.
Activists credit Obama with taking steps that will have the effect of reducing greenhouse gas emissions, such as issuing beefed-up vehicle fuel economy standards and proposing the first climate regulations for new power plants. But they also say the president hasn't met their expectations, for instance by failing to get cap-and-trade legislation through Congress and neglecting to make the threat of global warming clear to the American public.
Obama's accomplishments, while real, are far less than what it will take to head off a disaster for civilization, they warn.
The shift since 2008 has gone well beyond rhetoric. When Obama took office, he tapped a longtime Gore protégé, former EPA Administrator Carol Browner, to lead the White House's efforts on climate and energy.
But within two years she was gone after the bruising 2010 midterm elections.
Gore has emerged at times as a critic of the administration.
In a lengthy June 2011 Rolling Stone article, Gore took Obama to task, arguing that despite his achievements, he "never presented to the American people the magnitude of the climate crisis."
"President Obama has thus far failed to use the bully pulpit to make the case for bold action on climate change," Gore wrote. "After successfully passing his green stimulus package, he did nothing to defend it when Congress decimated its funding. After the House passed cap and trade, he did little to make passage in the Senate a priority."
Gore has also used his blog to express disappointment in Obama's decision last year to postpone toughened EPA regulations on smog, and to praise the protesters who "bravely participated in civil disobedience at the White House" to seek to block the Keystone pipeline.
The former vice president also pointed his readers last month to what he called an "important piece" in The Bulletin of the Atomic Scientists about how neither Romney nor Obama is focusing on climate change, despite the Western droughts and other extreme weather outbreaks.
But Gore is an even harsher critic of Romney and Paul Ryan, saying in a recent interview that "they've made statements that are just completely at odds with the facts, and they seem to be really in lockstep with the large carbon polluters."
In an article late last month, the New York Times said Gore has "moved on from the political world" and "is mostly at peace these days with losing the presidency in 2000."
"For Mr. Gore, Current TV is about living in the present; the cut and thrust of politics is the past," said the Times story, which added that Gore's "devotion to environmental advocacy … has sharpened his own outlook on what he wants to do with his life."
Gore declined an interview request from POLITICO through his spokeswoman. But supporters say it's not hard to discern his mood.
"Gore has been frustrated, as the rest of us have been, with the lack of action on climate," said Joseph Romm, senior fellow at the Center for American Progress and editor of the blog Climate Progress.
Romm, an Energy Department official under Clinton, expressed disappointment that Obama hasn't connected the nation's severe droughts to climate change.
"I've expressed great disappointment with the president for not pushing harder for legislation a few years ago and for not talking about climate change more, particularly since we've had this record drought, record heat wave and record flooding," he said.
Romm has long argued that the campaign and administration could score major political points by talking about climate change, arguing that top Obama officials are not talking about the issue because they're clinging to "misguided polling analysis."
But Romm, who just published a book on political communication, was quick to praise the administration's efforts to expand renewable energy and reduce harmful emissions from power plants and vehicles.
The administration finalized new rules last month that would require a 54.5 miles per gallon fuel efficiency standard by 2025. The regulation, which the administration has hailed as "historic," is the product of lengthy negotiations between industry and environmental groups.
Obama campaign spokesman Adam Fetcher said the president has made clean energy initiatives "a core economic pillar" of his administration — and that "by advocating for the growth of renewable energy, President Obama has continually called for action that will address the sources of climate change."
"While Mitt Romney also questioned the science behind climate change, President Obama will continue to make the case for cleaner American sources of energy that will create jobs and fight climate change," Fetcher added.
"Clearly, this is something that's important to the administration," campaign spokesman Tom Reynolds told reporters on a conference call in late August when asked why Obama rarely mentions climate change on the campaign trail. "But right now, we're obviously going to be focusing on jobs and the economy."
POLITICO and the Tampa Bay Times have partnered for the 2012 presidential election.