With Super Tuesday behind us, this week is the end of one chapter of this campaign. On the Republican side, former President Donald Trump’s only remaining challenger, Nikki Haley, is out of the race. And on the Democratic side, President Biden has so far secured more than 70 percent of the delegates he needs to secure the nomination. The general election is here. And so too is the rematch we’ve been expecting, despite the fact that the majority of Americans continue to say they wish they had other options. So for the next two episodes, we’re going to focus on a question we hear more than anything else: How exactly did we wind up with these two candidates? And why? First up: We map Mr. Biden’s path to the 2024 election through conversations with Elaine Kamarck, a longtime member of the Democratic National Committee and the author of “Primary Politics: Everything You Need to Know about How America Nominates Its Presidential Candidates,” and Ron Klain, the president’s former White House chief of staff.