This claimed by Catherine Rampell on the Washington Post ed page today, with a column entitled what is in quotes in the title of this post, with the answer being caveated as only being true after "the tweeter-in-chief," clearly the most dangerous man in Washington. Mulvaney is the Office of Management and Budget (OMB) chief who has been questioning the need to raise the debt ceiling and voted against doing so on a regular basis when he was in Congress. While Treasury Secretary Mnuchin wants a "clean" increase in the debt ceiling, Mulvaney has been saying since at least May that such an increase must be accompanied by "crippling spending cuts," unlikely to be accepted by Congress, thus threatening to bring a default.
On Sunday on Jake Tapper he sided with the declaration by Trump that nothing should be passed by the Congress until a Repeal and Replace of Obamacare is passed. Given that this does not look very likely at all, such a position also clearly signals a lack of concern about what might happen if the debt ceiling is not raised in time, which is probably mid-October at the very latest.
In an earlier discussion on this, some have argued that this might not be any worse than one of those "government shutdowns" we have previously seen, which did not amount to much. Social Security checks went out, the military kept operating, with aside from some bureaucrats in Washington being told to stay home the main thing the public saw was temporary closures of national parks. In contrast, Rampell argues that a failure to raise the debt ceiling would be followed by "a constitutional crisis, political and global financial crisis."
As it is, during the 16 day shutdown in 2014, according to OMB stats, about $2.5 billion was not paid to furloughed workers, although then paid later after the shutdown was over. However, something is off here, as that is equal to about 25 days of a $3.7 trillion overall federal budget. It is unclear how such programs as Social Security and Medicare were being paid, much less the interest on the national debt under these circumstances. Perhaps those 16 days were timed just right to catch two paychecks covering a 30 day period, which still does not quite cover things. Maybe this wonder can be repeated, but offhand I am skeptical, even if Rampell is overstating things.