The only certainty about the 2018 House elections is that Republicans will lose seats. This will not be the rare election like 2002, when Republicans gained eight seats, or 1998, when Democrats picked up five. This will be like the other 16 midterms since World War II, in which the president’s party lost seats.
How many seats will the GOP drop? The Democrats need 24 to win control. Here’s what to watch:
First, presidential job approval. A January Bloomberg analysis found that since 1970, parties whose president had an approval rating below 50% lost an average of 33 House seats in midterm elections. With President Trump at 40% in Gallup’s daily tracker and 42.4% in the Real Clear Politics average of recent polls, this indicator points toward a Democratic takeover of the House.
But wait. While low presidential approval tends to mean bigger gains for the other party, the relationship isn’t always predictable. In 2006, near the height of national angst over the Iraq war, George W. Bush’s approval rating was 38% and Republicans lost 30 seats. But in 2014, Barack Obama’s approval was 42%—roughly the same as Mr. Trump’s now—and Democrats lost only 13 seats. If the GOP matched that performance, it would hold the House.
Then there’s the generic ballot. Today, Democrats lead Republicans by 6 points in the RCP average, 44.4% to 38.4%—pointing to a close race for control of the House. Tax reform has contributed to a surge in GOP prospects: On Dec. 20, before it passed, the Democrats’ advantage was 13 points. But no one knows if tax reform will continue to deliver political dividends for Republicans. And given that the Democratic generic advantage has more than halved in the last six weeks, this measure will likely continue to oscillate.
Moreover, the relationship between generic-ballot polling and electoral outcomes can vary dramatically, as a Wall Street Journal analysis indicated last month. In 2006, Democrats were up by 10 on the generic ballot and picked up 31 seats; in 2008 they were up by 14 and added only 21. The GOP tends to add more seats with smaller generic-ballot polling margins: In 1994, Republicans were up by 4 points and gained 52 seats, and in 2010 they turned a 2-point lead into a 63-seat gain.
A Jan. 18 ABC News/Washington Post poll helps illustrate why generic ballot leads don’t go as far for Democrats. According to the poll the party’s edge on the generic ballot is 38 points in districts it now holds. The GOP advantage in districts it holds is just 6 points. Concentrated Democratic margins in already safe Democratic seats provide little help in winning Republican-held districts.
Further complicating this year’s midterm is what political scientists call “surge and decline.” In presidential elections, the White House winner often brings in new members in marginal seats, who then lose in the midterm. When Ronald Reagan won in 1980, 34 new Republican congressmen rode in on his coattails. Twenty-six didn’t survive the next midterm. Not every new president’s election creates a surge, however. When Mr. Trump won in 2016, Republicans lost six House seats. This suggests there might be fewer freshmen vulnerable to “decline” this November.
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The most important midterm variable, however, is incumbency. All but 15 of the 219 incumbent GOP congressmen ran ahead of Mr. Trump in the 2016 election. A National Journal analysis found that 93.8% of incumbent congressmen who sought re-election since 1980 won. For Republicans to keep the House, they must minimize retirements among the 23 Republicans whose districts were carried by Hillary Clinton.
Of the 48 House members who have announced their retirements so far, 33 are Republicans and 15 are Democrats. Six Republicans are in districts Mrs. Clinton won, and two more are in districts Mr. Trump carried by 1 to 4 points. These are the seats most at risk this year. But the 15 GOP-held seats opening up in districts Mr. Trump carried by 20 to 49 points, and most of the 10 seats where his margin was 5 to 19 points, are likely to remain Republican.
Three other important factors are money, enthusiasm and opposition candidate quality. These are worthy of in-depth attention in another column. For now, suffice it to say that indicators are giving off ambiguous signals with most of this election game to go.