What If an Alternative Presidential Candidate Is Needed in 2024?

From a New York Times column by David Brooks headlined “If an Alternative Candidate Is Needed in 2024, These Folks Will Be Ready”:

What happens if the 2024 election is between Donald Trump and somebody like Bernie Sanders? What happens if the Republicans nominate someone who is morally unacceptable to millions of Americans while the Democrats nominate someone who is ideologically unacceptable? Where do the millions of voters in the middle go? Does Trump end up winning as voters refuse to go that far left?