The Great Switch by the Super Rich makes a point I hadn’t fully understood. 40% of US national debt is owned by Americans, much by the richest 1% (though the article does not provide figures).
This means that the richest people in US society have reduced the amount they pay in taxation and increased the amount they lend the government (which they get paid for).
In addition, the budget deficit is made more concerning, because all holders of Treasury bills (in the US and abroad) might dump them if they get nervous.