Carter addressed the DNC by video on Tuesday. He is 91 years old and only recently made a full recovery from cancer, so his physical absence should not be surprising. He did say that he plans to campaign for Hillary Clinton in the fall. But there may be another reason for his absence: He and Bill Clinton just don’t like one another very much.

In his short address, Carter mentioned that he and the Clintons go way back—he appointed Hillary to the board of the Legal Services Corporation in 1977 and she became chair in 1978. Things were fine between them until 1980, when Bill Clinton lost the governorship of Arkansas and Jimmy Carter lost the presidency. Clinton blamed his loss on Carter letting 19,000 Cuban refugees come to Arkansas. Over the years that followed, Carter seemed threatened by his fellow charming Southerner of humble origins, while Clinton bristled when he was compared to the one-term president.

When Clinton ran for president in 1992, Carter didn’t endorse him, saying, “People are looking for somebody who is honest and tells the truth.” Clinton hit back by refusing to acknowledge him at his 1992 inauguration. Things continued to fall apart from there. Brought together again in 1995, Clinton once again refused to acknowledge Carter. In 1996, Carter skipped the DNC. When Clinton pushed to bomb Iraq in 1998, he reportedly said, “Carter will probably criticize me. Carter always criticizes, but he doesn’t have much positive to say.” In 2008, Carter snubbed Hillary Clinton and endorsed Barack Obama.

The two seem to have patched things up a bit—they appeared together in June to discuss the election and slam Trump—but at best, they seem to barely tolerate one another. Carter’s appearance at the DNC may have more to do with Donald Trump than anything else.