Nathaniel Rakich and Humera Lodho explain why “Why Democrats’ Midterm Chances Don’t Hinge On Biden’s Approval Rating” at FiveThirtyEight:

Earlier this month, FiveThirtyEight editor-in-chief Nate Silver noted an interesting disconnect between two pieces of information most commonly used to predict the upcoming midterms: the president’s approval rating and polls of the generic congressional ballot (which ask Americans whether they plan to vote for the Republican or the Democratic candidate this fall).

On one hand, President Biden is historically unpopular: As of July 25 at 5 p.m. Eastern, he had an average approval rating of 38 percent and an average disapproval rating of 57 percent — a net approval rating of -19 percentage points. You have to go back to Harry Truman to find a president with a net approval rating that bad at this point in his term.

On the other, generic-congressional-ballot polls are pretty close. As of the same date and time, Republicans had an average lead of 1 point.

“Those two numbers feel difficult to reconcile. Biden’s approval rating suggests that the national mood is extremely poor for Democrats, while the generic-ballot polling suggests that the political environment is only slightly Republican-leaning. But in reality, these two types of polls aren’t in opposition as much as you might think. They’re separate metrics, and a look back at past midterm elections shows they don’t always line up. But history also shows that when they do diverge, one is more predictive than the other.

Rakich and Lodhi note further, that”plenty of Democrats tell pollsters that they disapprove of Biden’s performance, but almost all of them also say in the same breath that they will vote Democratic in the midterms (that is, if they turn out to vote — an important caveat).” Also, “it’s not unusual for presidential-approval polls and generic-ballot polls to disagree. Just take a look at where the polls stood on July 25 in the past four midterm election years: 2006, 2010, 2014 and 2018.^1“

They review the historical data in more depth and ask, “So that leaves us with one final question: Which of those two indicators should we be paying more attention to?” Their answer:

The answer is the generic ballot. Unsurprisingly, polls asking Americans which party they plan to vote for in the midterms have historically been more predictive of the midterm results than polls asking about presidential approval. As Silver concluded, the president’s popularity just doesn’t add all that much new information when you have polls that directly ask the question you want answered….In the past four midterm elections, the generic-ballot polling average has missed the national popular-vote margin for the House of Representatives by an average of only 2.5 points, while the presidential-approval polling average has “missed” (we’re using scare quotes because presidential-approval polls are not intended to be measuring this) the national popular vote margin by 5.5 points. In each of those cycles, regardless of whether the two numbers were in sync or not, the generic-ballot polling average came closer to the final vote margin — sometimes significantly closer.”

But don’t uncork the bubbly just yet, because Rakich and Lodi write, “The generic-ballot polling got worse for the president’s party in all four cycles….a trend that’s especially pronounced when a Democratic president is in office….by the fall they will be conducted among likely voters — a group that will probably be disproportionately Republican, both because Democrats tend to be more infrequent voters in general and because, currently, more Republicans than Democrats say they are enthusiastic to vote.”

They conclude: “So Republicans may lead in generic-ballot polls by only 1 point on average today. But by November, their lead will probably be a few points wider. And while that wouldn’t be as disastrous for Democrats as it would be if everyone’s midterm vote was dictated by how they rated Biden’s job performance, it would still be a great result for Republicans.”

Could this year be different because of Trump, Covid or weak GOP Senate candidates? Rakich and Lodi apparently doubt it, since they don’t address the possibility. It doesn’t seem too much to hope that one of theses factors could make a small difference for the better. But every recent midterm election in their study has had its unique twists and turns, and not many get rich betting against such voting patterns in politics.