“Wave” elections usually build slowly in the distance. But we are seeing some of the same indicators that portended big shifts in 2006 and 2010. Photo: Panoramic Images/Getty Images

Midterm elections are almost always a perilous time for parties that control the White House. The president’s party has lost House seats in 15 of the last 17 midterms, and the opposition party counts on the midterms as a staging point for a comeback. When a party controls both the executive and legislative branches of the federal government, as the GOP does today, a midterm loss can be highly disruptive. If, as seems possible, Democrats win the net 24 House seats necessary to take away Paul Ryan’s gavel and give it back to Nancy Pelosi, the Trump administration’s already sizable difficulties in enacting legislation would metastasize.

Let’s start with the caveat that odds around what will happen in 2018 are hard to calculate. On the one hand, there are multiple signs the president is maintaining the support of his hardcore base. And Trump and his party took a major step towards solidifying that base by finally getting legislation repealing Obamacare out of the House. On the other hand, looking beyond the base, Democrats seem to have an enthusiasm advantage (as reflected in their candidates’ relatively strong performance in special elections so far, such as those in Kansas and Georgia). The big legislative product of the Trump Era so far, the American Health Care Act, is very unpopular to the extent people understand it, as reflected both in polls and in the hostile environment Republicans are encountering in meetings with constituents. And the firestorm over Trump’s firing of FBI director James Comey is probably not going to help the GOP.

That said, veteran political prognosticators are beginning to smell the salty air of a potential Democratic “wave” election in 2018 — possibly large enough to net the 24 seats needed to depose Paul Ryan as Speaker. Here’s Charlie Cook:

Roughly once a dec­ade we see a tid­al wave elec­tion, al­most al­ways at midterm, in which an in­vis­ible hand seems to boost can­did­ates of one party and drag down can­did­ates of the oth­er. Can­did­ates who nor­mally win big end up win­ning by smal­ler mar­gins. Law­makers who usu­ally have com­pet­it­ive races of­ten get sucked away by the un­der­tow. Dis­tricts that should be safe are no longer safe. Strong cam­paigns lose to weak cam­paigns, un­der­fun­ded cam­paigns topple well-fun­ded cam­paigns.

Electoral waves tend to build slowly. In the last really big wave election, in 2010, an awful lot of Democrats who looked safe at the beginning of the cycle wound up in the crosshairs, and in the end, Republicans gained a shocking 63 net seats. But there are usually omens. Cook’s ace House analyst David Wasserman recently moved 20 Republican-held seats into more vulnerable territory in recognition of the pro-Democratic dynamics developing. Democrats have been regularly leading in measurements of “the congressional generic ballot,” a polling question that simply asks which party should control the House.

A new poll shows the kind of numbers that if they become common could definitely portend not just a “wave” but a veritable tsunami. Quinnipiac’s latest national poll mainly drew attention for showing some really terrible assessments of Donald Trump. But its congressional generic ballot was a shocker:

By a 54 – 38 percent margin, American voters want the Democratic Party to win control of the U.S. House of Representatives. This is the widest margin ever measured for this question in a Quinnipiac University poll, exceeding a 5 percentage point margin for Republicans in 2013.

Indeed, I could not find any polls showing that kind of margin for either party during the 2014 or 2016 cycles (there was one Rasmussen poll in late 2013 — when Republicans were getting blamed for a government shutdown — that showed Democrats up by 11 points, but no other double-digit leads were evident going into either the 2014 or 2016 elections).

If you go all the way back to 2010 or 2006 — two genuine wave elections — double-digit leads for one party in the congressional generic ballot were common. The final Real Clear Politics polling average in 2010 showed Republicans up by 9.4 percent; their actual national-popular-vote margin was 6.8 percent, which yielded (thanks to the overexposure caused by two consecutive big Democratic wins) a net gain of 63 seats for the GOP. In 2006, the final RCP average was even larger for Democrats: 11.5 percent. Democrats actually won the national popular vote by 7.9 percent, picking up 31 net seats.

So there is at least some reason to believe a big shift along the lines of 2006 or 2010 could be in the works, particularly given the president’s persistently poor approval ratings. The Quinnipiac poll may be an outlier, but then again, it is reminiscent of the 2006 exit polls: It shows Democrats actually leading among seniors, a group the Donkey Party won in 2006 and 2008 but has lost consistently ever since (by more than 20 points in 2010 and 16 points in 2014).

No one knows what kind of popular-vote margin Democrats would need to retake the House, given the advantages Republicans have via gerrymandering, superior vote distribution, and incumbency. But in February, Harry Enten made a credible rough guesstimate:

Since 2012 (or when most states instituted the current House district lines), Republicans have won, on average, 51 percent of the two-party House vote and 55 percent of House seats. If that difference holds for 2018, Democrats would need to win the House popular vote by about 8 percentage points to win half the House seats.

Sixteen points would far more than do the trick. Paul Ryan’s grip on that gavel is looking a bit slippery.