As much trouble as President Jimmy Carter was in politically in 1979-80, having an approval rating somewhere in the thirties and looking very vulnerable to Ronald Reagan in the 1980 presidential election with all of the economic problems and the Iranian Hostage Crisis, President Carter whipped Senator Ted Kennedy in most of the Democratic presidential primaries. Senator Kennedy didn't win many if any states outside of the Northeast in 1980.
With a better stronger more personally disciplined presidential candidate with the same popularity in the Democratic Party as Ted Kennedy had, President Carter would've been a lot more vulnerable and perhaps isn't renominated for president. But Ted Kennedy turned out to be a lousy presidential candidate for a few reasons.
He didn't want to be president and was running out of some obligation he believed he had to the Democratic Party.
He didn't even know why he wanted to be president as Roger Mudd made clear in the CBS interview.
And he had a lot of personal issues that he was dealing with at that time. Like getting divorced and rumors around he was involved with multiple women.