One clear troubling sign for the Democrats at this early stage is independent voters, who decide most elections. They are evenly divided, according to Pew’s mid-October survey: 43% say that “if the elections for Congress were being held today,” they would vote for the Republican candidate in their district, 43% say they would vote for the Democratic candidate.

It is not too much of an oversimplification to say that Democrats are struggling because President Obama is struggling.

At the start of Mr. Obama’s second term 52% approved of the job he was doing, 40% disapproved in the Pew Research January national survey. His job-approval ratings reversed by October: 43% approved and 51% disapproved. In Thursday’s Gallup survey, Mr. Obama’s job approval was 42%

The economy and ObamaCare’s inauspicious debut are likely the most powerful drags on the president and in turn on his party. In a September Pew survey, 63% of Americans say the nation’s economic system is no more secure today than it was before the 2008 market crash.