Gore’s reticence, his friends and allies say, is in part to maximize his own leverage on fighting climate change. But his repeated demurrals also reflect a complicated relationship with his former boss’s wife that dates back more than two decades. While Gore and Hillary Clinton may not be enemies, they’re not exactly close friends, either.

“They have a lot of history. More than the average bear,” said one Democratic source close to Gore.

The Hillary Clinton-Gore rivalry started when the two baby boomer policy wonks arrived in the White House in 1993. Gore got the new administration’s environmental and technology portfolios. Bill Clinton raised eyebrows by assigning his wife what would become an ill-fated attempt to pass comprehensive health-care reform legislation.

“Usually you give your vice president something of that level. You don’t give it to the first lady,” recalled a former Clinton White House staffer. “People forget that sort of started the relationship on a downward spiral early on.”…

“Her Senate race complicated things,” another former White House aide said. “It took up a lot of the political oxygen and party money. It put the spotlight on the Clintons when it wasn’t focused on his race. It didn’t help.”