An excellent summary from James Hamilton four years ago that's just as appropriate now:
One of the peculiar embarrassments of the American political process is the fact that Congress votes separately on the deficit and debt, as if they were two different decisions. This bizarre arrangement allows Congress the luxury of instructing the Treasury to spend more than it takes in as revenue while at the same time voting to deny the authority to borrow the funds that would be necessary to implement the plan.
If the government is (a) required by the deficit legislation to spend, and (b) precluded by the debt legislation from borrowing, the Treasury would be forced into default. The greater the likelihood markets attach to such an event, the higher will be the interest rate the government has to pay on Treasury debt. A politician who votes for the spending and tax measures that produced the deficit but against a debt ceiling consistent with these is deliberately wasting taxpayer dollars for no purpose other than to grandstand before voters as a "fiscal conservative". Anyone playing such a game has complete contempt for the intelligence of their constituents.
I don't have a problem with Republicans using the opportunity of raising the debt ceiling to demagogue the deficit problem--this is one issue that needs all the passion it can get. And Democrats can hardly complain, since one Senator Barack Obama was doing just that four years ago.