PHILADELPHIA — Barack Obama sweeps into the Democratic National Convention Wednesday with an opportunity no president has had in nearly three decades: to help his chosen successor. And make no mistake about it, Hillary Clinton needs his help. She struggles with getting voters to like her, recent polls show she’s vulnerable against Republican Donald Trump, and more than a few Democrats attending their national convention still prefer her primary season rival, Bernie Sanders. Clinton has embraced Obama, saying she wants to build on his vision – a marked difference from past years when presidential hopefuls have run away from their party’s presidents because of personality clashes or low approval ratings. It is the first time since 1988 that an outgoing president has been popular enough to help and the first time a president’s party has nominated someone who genuinely wanted the help. “A president’s popularity always helps someone running for his party in a presidential year,” said Ken Duberstein, who served as Reagan’s chief of staff in his second term. Obama will not help Clinton win over Republicans or conservative-leaning independents, but, political experts say, he can help bring Sanders’ supporters on board and persuade Democrats to get to the polls in November. “A president is never going to be extremely popular in this age of polarization,” said George Edwards, a presidential scholar at Texas A&M University.