From “Democrats Have the Biggest Midterm Election Lead in May, by any Party, in 20 Years:  All signs point to a Blue Wave, and they may need every drop to overcome recent gerrymandering woes and likely presidential scheming” by Bill Scher at Washington Monthly:

While the polling industry has taken a lot of guff in recent years, one question continues to earn its keep: the generic congressional ballot test. It is a poll question that asks respondents which party’s candidate they plan to support to represent their House district. The average lead in the general congressional ballot test can’t tell us exactly how many seats will flip, but it can tell us if a wave is coming.

Three of the last five midterm elections have been “wave” years in which the party that did not control the White House flipped over 25 House seats—the post-World War II average—en route to seizing the Speaker’s gavel. In each of the wave years—the 2006 and 2018 Blue Waves, as well as the 2010 Red Wave—the Real Clear Politics average generic congressional ballot test lead for the opposition party cleared five points. At no point in the non-wave years of 2014 and 2022 did either party meet that mark (though Republicans came close in 2022, briefly touching 4.8 in April).

This year, Democrats have already cleared the five-point mark in the RCP average, first for a few days in February, then again in late March. This spring, the Democratic lead has steadily ticked upward. Buoyed by particularly strong data in the latest New York Times poll, the Democrats’ lead stood at 7.2 points as of May 18. That nearly matches the 7.3-point poll lead Democrats had before Election Day 2018.

That’s significant for two reasons. One, no party has built an average seven-point generic congressional ballot test lead in RCP this early in a midterm year since 2006, when public ire over the Iraq War quagmire, the botched response to Hurricane Katrina, and the unpopular and unsuccessful push to privatize Social Security partially fueled double-digit Democratic polling leads from the very beginning of the year.

Two, in the RCP average, no party that led by seven points at some point during a midterm year ever ended the election year with a lead of less than seven points. In the more crucial metric of the aggregate House popular vote margin, that’s also pretty much true, although the 2010 Republicans scored slightly below at 6.8 points.

Of course, there can always be a first time. Perhaps President Donald Trump gets the Strait of Hormuz reopened, which eases pressure on gas prices and prompts a voter reassessment. But most energy industry experts have cautioned that even with a resolution with Iran, prices may not return to pre-war levels, partly because of damage to energy infrastructure. More likely, we are seeing a dynamic familiar in wave years—year-long, stable polling leads for the opposition party. (An exception was in 2010, when Democrats were competitive in generic ballot polls through June before Republicans broke away.)

More here.