Republicans will nod their heads and pump their fists when Trump unloads both 12-gauge barrels on Biden, charging that he is senile, fragile, doddering, weak, confused, unsteady, unreliable, and at the center of a family network of rapacious political profiteering. But don’t expect that to be a knockout punch, even if you think it should be — Republicans are not Trump’s problem: Democrats and independents are.

All Biden really has to do is mend fences with the Sanders gang. Trump has a steeper hill to climb. In 2016, he won critical victories in Florida, Pennsylvania, Michigan and Wisconsin — but in none of those states did he break 50 percent. Trump’s margin of error is tiny.

The line that Trump is sure to deploy against Biden already has been tried in the primary — and Trump should take note: It did not work, in spite of the best efforts of Sanders’ surrogates who are every bit as vicious as Trump can be. Charging that Biden is senescent did not work on Democrats in Michigan or Minnesota, and it is not working in Pennsylvania, where Biden leads Sanders by 25 points in the polls — and where he leads Trump by 4.

Complaining about Biden’s gaffes is like complaining about Trump’s spelling or his adultery — it’s nobody’s news flash.