Professor Stephanie Kelton (UMKC and economic adviser to Bernie Sanders) discussing what the limit is on how much a currency-issuing government can spend. The limit is not "running out of money," or "investor confidence in ability to pay back debt." The government cannot run out of the currency it issues, and it can never be forced to default on its debt (which it doesn't even need to issue).
Modern Monetary Theory (MMT) is gaining traction in American politics, energizing the progressive left and roiling deficit hawks. Stephanie Kelton, who advised Bernie Sanders’ 2016 presidential campaign, explains the basics.
Professor L. Randall Wray discussing how bond sales work with a currency-issuing government with a floating exchange rate. Because the government can issue currency (and indeed must every time it spends) there is no need to issue debt in order to spend.