The 2018 election is getting real as September arrives. Photo: John Lund/Corbis via Getty Images

Labor Day is often regarded as marking the beginning of the most serious phase of any general election cycle, as voters slowly start to focus on politics after summer diversions fade and the major parties place their bets by choosing where to invest their resources. There’s no guarantee that polling trends in early September will prove more predictive than those in early August. But midterm “wave” elections do tend to build strength late in the cycle. And there are some initial signs that that could be happening to the benefit of Democrats even before the climate-change-driven weather begins to turn cooler.

The single most important indicator of national popular vote trends in a midterm election cycle is the generic congressional ballot, a polling question that simply asks respondents which party they favor in House elections. In the current cycle much of the early talk of a Democratic “wave” stemmed from generic Democratic generic ballot advantages in late 2017 that reached double-digits (according to the polling averages at RealClearPolitics, the Democratic advantage peaked at 13 points on Christmas Day). In the current calendar year, though, that advantage has shrunk, reaching a low (again, according to RCP) of 3.4 percent in May. More recently, Democrats have slowly rebuilt a generic ballot edge in the neighborhood of the 6-8 percent most analysts think might enable them to retake control of the House.

But now that advantage is again swelling, standing at 9.5 percent at RCP, and at 10.8 percent in the averages at FiveThirtyEight, where raw data is adjusted to reflect pollster reliability and demonstrated partisan bias. And that’s probably an actual trend, not a blip: five of the last six polls in the RCP averages show Democrats holding edges ranging from 11 to 14 percent.

The other much-cited national poll, the president’s job approval rating, has been more stable in 2018, after lagging in the last few months of 2017. Again using RealClearPolitics’ polling averages as a benchmark, Trump’s approval ratings breached 40 percent in the first week of January, and only briefly dropped below that mark in March. His approval ratings right now are at 41.8 percent at RCP and at 40.1 percent at FiveThirtyEight. Does that contradict the apparent Democratic surge in the generic ballot?

Not really. What it really means is that Trump’s approval rating is almost certainly not going to rise to the levels that in the past have insulated the party of the president from significant House losses in midterms, especially first midterms after a president has taken office. If he’s very, very lucky, Trump might reach an approval rating of 45 percent by election day (though that’s unlikely since he hasn’t been at that level since the first days of his presidency). As it happens, 45 percent was the final pre-midterm approval rating of both Bill Clinton in 1994 and Barack Obama in 2010. Clinton’s party lost 54 House seats; Obama’s lost 63.

The generic ballot trends parallel another telling data point: the very lop-sided and growing partisan disparity in individual House districts at risk, as the Cook Political Report’s David Wasserman explained in late August: