If 2006 and 2010 patterns hold, a Democratic midterm wave could appear late in the day as vulnerable GOP House seats become targets. Photo-Illustration: Elenarts/Getty Images Plus (Donkey); John Lund/Blend Images/Getty Images (Surf)

At the moment, conventional wisdom has it that the Democratic Party’s odds of reclaiming control of the House are on something of a knife’s edge. The Democratic lead in the generic congressional ballot is standing right on the cusp of the 7- to 8-percent lead most analysts consider necessary to flip the 23 seats needed for control. Democratic enthusiasm remains high. But it’s hard to figure what dynamics will matter most in the stretch run.

The Cook Political Report’s Amy Walter points out that in two recent “wave” midterm elections, in 2006 and 2010 — both of which flipped control of the House — the late trends were very strongly against the party controlling the presidency:

﻿In July of 2006, The Cook Political Report rated just 14 GOP-held seats as highly vulnerable. By November, the number of GOP-held seats in danger had tripled to 43. We saw a similar pattern in 2010. In August of that year, we listed 36 Democratic-held seats as highly vulnerable. By November, the number of vulnerable Democratic-held seats had more than doubled to 78. On Election Day of 2006, Republicans lost 30 seats; Democrats lost 63 seats in 2010.

A lot of seats that wound up falling weren’t even on the radar a few months earlier:

[O]f the 30 seats that Democrats won in 2006, 21 of them (or 70 percent), weren’t classified as the most vulnerable GOP-held seats in July. Almost half of the Democratic seats Republicans won in 2010 were classified as Lean or Likely Democrat in August.

Right now, Cook has 34 Republican-held House seats looking very vulnerable (3 are likely Democratic, 7 lean Democratic, and 24 are toss-ups). But the landscape could get much bluer in a hurry:

This year, Republicans already have more seats in the highly vulnerable category than they had at this point in 2006 or than Democrats had in August 2010. If 2018 follows a similar pattern to 2006 and 2010 — where less vulnerable seats move into more vulnerable territory in the fall — the GOP is almost certain to lose its majority. There are currently another 53 GOP-held seats in lean/likely Republican.

What would account for this kind of late trend? In 2006 and 2010 it was not, interestingly enough, any deterioration of the president’s own approval ratings:

There wasn’t a point where the bottom just dropped out for one party. The approval rating for President George W. Bush was 40 percent in mid-July 2006 and 38 percent in early November. President Obama was sitting at 44 percent in mid-August 2010 and 45 percent in early November.

So Donald Trump’s exceptionally stable approval ratings won’t necessarily serve to limit his party’s losses in the House. Late trends could also reflect intensifying excitement over an approaching win for the “out” party. But most likely what we are seeing is simply a public recognition of trends that were developing all along. Forecasters like those at Cook are naturally conservative about predicting change of party control for any given House district, given the power of incumbency and the generally strong grip of partisan inclinations in this century.

There have, of course, been some nasty surprises for the “out” party in late midterm trends, and they’ve been relatively recent. In 1998 when Democrats actually gained House seats in Bill Clinton’s second midterm (Newt Gingrich made his painful concession-of-failure speech in front of a backdrop covered with the legend “America’s Victory”), and in 2002 when Republicans repeated that amazing feat. But then Clinton in 1998 had a approval rating just before the midterms of 66 percent, and George W. Bush enjoyed a 63 percent approval rating going into the 2002 midterms. Add in the reaction to the pending GOP impeachment effort in 1998 and the effects of 9/11 on 2002, and you have sets of circumstances that are extremely unlikely to recur between now and this November.

We could even realize in retrospect that the best 2018 signals to watch may have been the strongly pro-Democratic special-election results in 2017 and 2018, rather than November projections or even the generic ballot, as portending a wave that has been obscured in those less-tangible indicators. Since the pace of special elections has slowed this year, a lot of attention will be focused on next week’s special election in Ohio over yet another GOP-held U.S. House seat (one that Republicans have held for 36 years).

In any event, it will all get very real in the fall, and if both parties seem to be achieving new levels of furious intensity, it could be because the electoral tremors beneath the surface are getting stronger every day.