CHARLESTON, S.C. — The brother who was never expected to become president — but did — will be trotted out Monday as a lifeline to the brother who was always expected to become president, but now may not.

Former President George W. Bush will take the stage Monday evening alongside Jeb Bush for a highly anticipated rally in North Charleston, S.C., part of the Bush team’s effort to use the primary next Saturday in South Carolina — a state long favorable to the Bush family — to help reinvigorate Mr. Bush’s stalled presidential bid. The Bush campaign is also hoping the rally can seize on what was regarded as a strong debate performance on Saturday night.

But the appearance by the brothers — one the 43rd president, the other fighting to become the 45th — also offers a glimpse into the complicated dynamics of a family dynasty, as well as striking parallels with George Bush’s 2000 campaign here. It was when South Carolina was similarly emerging as a critical state for him, and where the race took a darkly negative turn.

“I’m proud of the fact he’s coming, and honored,” Jeb Bush, a former governor of Florida, told reporters on Thursday. “This is the first that he’s really kind of stepped out in the political realm since he was president.”