Sen. Kamala Harris of California scored a tremendous political moment last week when she lashed out at former vice president Joe Biden over his record on school busing for desegregation during the first Democratic presidential debate in Miami, Fla.

But one might imagine that such rough exchanges among fellow Democrats will only serve ultimately to weaken the ultimate nominee once he or she must face Donald Trump next year.

Just the opposite is true.

Because if they don’t talk about it, you bet that Trump will. And he’ll talk about things no Democrat will talk about at the debates, like Harris’ history with the late San Francisco mayor, Willie Brown. Yes, it’s 90 percent bull, but it’s effective bull. And we need to be prepared to deal with it.

There’s a huge difference between personal attacks — I don’t think people should be bringing up Willie Brown, or the Native American business in the primary debates — and issues. Bringing up Biden’s civil rights record is legitimate, as would be bringing up Harris’ record as a prosecutor.

Those things don’t invalidate the different positions they’re taking now, but people deserve an explanation for the change. A candidate who doesn’t have one will be (rightly, in my opinion) perceived as opportunistic and false.

I don’t expect people to support every position they took 20 years ago, but I expect them to have a reasonable story about their evolution away from them.