You may not be old enough to recall or to follow as far back as LBJ, or even Reagan, but Presidents who claim a voter mandate are given significantly more leeway when they push their respective agendas. In the case of LBJ and Reagan, I would suggest that they made mistakes because they operated under a false sense of confidence.
LBJ pushed his Great Society agenda in the emotional aftermath of a Presidential assassination rather than an electoral mandate. Nevertheless, Congress was very reluctant to resist items that were clearly part of Kennedy’s platform. Including the push back against Communism which resulted in the quagmire that was Vietnam.
In Reagan’s case, his early efforts to spend his way out of a recession was initially successful, and an electoral mandate in 1984 inspired him to continue on the path that ultimately undermined some of the gains and forced George Bush to reneg on his promise of no new taxes.
By contrast, when Bill Clinton was saddled with midterm election losses, he was forced to work with a GOP congress to avoid gridlock. To his credit, he did so. He was still pushing programs and agendas even during his last tow years.
In conclusion, I may not necessarily be right about everything, but I am basing my ideas on concrete examples.