The door slammed shut on Barack Obama’s legislative ambitions in November 2010. Enraged at being out of power and at the passage of a liberal health-care bill, Republicans swarmed to the polls in the mid-term elections. This cost the Democrats their majority in the House of Representatives. Mr Obama had to rely on executive orders for his final six years in office.
In 2018 the tables will be turned. Now Republicans hold all three elected arms of the federal government, and must brace themselves for voters’ wrath. Will that return the House Speaker’s gavel to Nancy Pelosi?
The starting-point for predicting the results of House elections is the “generic congressional ballot”, a question frequently asked by pollsters: respondents simply name the party for which they plan to vote. This has a stunningly accurate record at forecasting the overall national vote for the chamber. Over the 38 congressional elections since 1942, a simple statistical prediction based on the final polling average, and adjusted for whether the cycle was a midterm or a presidential year, has missed the target by an average of just 1.3 percentage points. The generic ballot also tends to stabilise fairly early in an election cycle, and is particularly informative when combined with data on presidential popularity, the economy and voters’ loyalty to their preferred political party. Using a broader model based on all of these factors, the Democrats’ current advantages correspond to a victory by five or six percentage points come November 2018.
Even that comfortable margin would probably not be sufficient to flip either chamber of Congress. In the Senate, the Democrats face a brutal map. Of the 33 senators up for re-election, 25 are Democrats or independents who caucus with them, many of them in deep-red states. A “blue wave” might enable all 25 to hold on, as well as knocking off vulnerable Republican incumbents in Nevada and Arizona. Yet that would still leave the GOP with 50 seats, enough to maintain control thanks to the tiebreaking vote of Mike Pence, the vice-president.
To wrest away the upper chamber, Democrats will also have to win a seat held by a Republican in a solid red state. The leading candidates are probably Tennessee, where Bob Corker is retiring, or Texas, where the ultra-conservative Ted Cruz faces a strong challenger in Beto O’Rourke. But punters on the PredictIt betting market give Republicans a nearly 80% chance of holding on.
In the House, all 435 seats are up for grabs and Democrats have to win back 24 of them to regain control. Yet to do so they need to overcome both gerrymandering (which favours Republicans) and the tendency of Democratic voters to cluster in liberal cities, giving them lopsided margins in those districts. So Democrats may need to win the national vote by a whopping seven points—a margin consistent with historic landslides like 1994, 2006, 2008 and 2010—just to eke out a victory by a single seat.
The Democrats can counter with an edge of their own on incumbency. Perhaps because of unfavourable political winds, numerous Republican representatives in close or Democratic-leaning districts have decided to retire. So far, no Democrats in vulnerable seats have followed suit.
Still, the odds suggest that Democrats could fall just short. Bettors on PredictIt give them a 48% chance of taking the House. A model on Decision Desk HQ, a website, that combines polling, incumbency and each district’s partisan lean, puts it at 46% (see map).
Control of the House matters. Even if Democrats steered clear of impeachment, they would gain the ability to launch investigations into Donald Trump’s dealings. The fate of Mr Trump’s presidency could depend on just how resilient his party’s gerrymanders are to an unforgiving mid-term electorate.
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