The most likely consequence of investors doubting the federal government's ability to repay the national debt would be:
Fewer investors would be purchasing the securities from the Treasury, resulting in an increase in interest rates, and a further increase in the cost of borrowing for the national debt.
When investors lose confidence in the government's ability to meet its debt obligations, they may demand a higher return (interest rate) to compensate for the perceived risk. This can lead to higher borrowing costs for the government and potentially exacerbate the situation.