Former Vice President Al Gore said that "obligations in Tennessee" will keep him from attending the Democrat National Convection in Philadelphia, where his former political partner's wife is set to receive the Democrat's nomination for president.

Gore remains one of the last Democratic Party luminaries not to make a formal endorsement of Hillary Clinton. Betsy McManus, Gore’s communications director, told POLITICO last fall that he’d wait until a nominee had been formally selected to weigh in. But it’s been more than a month since Clinton sewed up the necessary delegates, and Gore has maintained his silence on the 2016 presidential race.

Gore's friends have said that the former Vice President was keeping quiet in order to make climate change "a more potent issue with the presidential candidates and also because of his complicated relationship with the former first lady that dates back more than 20 years."

Philadelphia isn't the first DNC that Gore has missed; he also skipped the 2012 DNC in Charlotte, North Carolina. He was anchoring coverage for his network CurrentTV, which was later sold to Al Jazeera.

“His world seems to be always full and outside the political world,” said Kiki McLean, a spokeswoman for Gore’s 2000 presidential campaign.

Gore was also too busy to grant an interview to Politico. “He’s not available this week,” McManus wrote in an email.

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