Americans are usually forgiving when they vote a man into the White House and he wants a second term. Of the last eight elected presidents, all but two — George H. W. Bush and Jimmy Carter — got their four more years. Which is why the conventional wisdom long held that Barack Obama would most likely weather his midpresidency slump to win another term.

Then came the debt-ceiling debates of July and August, which seemed to crystallize Obama’s vulnerabilities in a way that even the Democrats’ midterm disaster of 2010 did not.