Former political professional here. The truth is that the American people were never that concerned with the national debt. As a fiscal issue it is very abstract, future-oriented, and difficult to understand. Most voters understood it culturally: politicians who discussed it effectively communicated to voters that they cared about taxpayer money, and future generations.
But fiscally? It doesn’t decide an appreciable number of votes. It never did. That Republicans focused so much on it was a sign of their own fixation of seeing government as purely a matter of dollars and cents, rather than what it has always been, expressing a particular culture of governance to voters that gets them to believe in what you want to do.