Don't expect much of a post-convention bounce for Trump or Clinton
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One of the most regular rhythms of recent presidential politics has been the "post-convention bounce" that each nominee receives in the polling after their convention. However, the size of the bounce has been declining recently; given the pairing of GOP nominee Donald TrumpDonald John TrumpTrump administration eyes proposal to block jet engine sales to China: report Trump takes track to open Daytona 500 Brazile 'extremely dismayed' by Bloomberg record MORE and presumptive Democratic nominee Hillary ClintonHillary Diane Rodham ClintonThe 'Palmetto Promise': South Carolina will decide the race Alabama Senate contender hits Sessions in new ad: 'Hillary still ain't in jail' Worries grow as moderates split Democratic vote MORE and their current standing in the electorate, it is likely to be negligible in 2016.

What is the post-convention bounce?

The bounce is a term used to describe the difference in the polls conducted just after the convention compared to those just before, using the trial heat question about who would you vote for. At this point in the campaign, journalists will focus on Trump's standing in the polls (his popular support) rather than the difference between his support and Clinton’s (the lead of one candidate over the other), although these two measures are related to a certain extent.

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Recently, it has been calculated as the difference in the average of a set of polls conducted before and after the convention. One problem is the polls used to calculate the averages can be different in the two measurements, and that can be a complication. Because of differences in the way the polls are designed and conducted by different polling firms — what survey researchers call "house effects" — some of the difference can be due to methodological artifacts.

This can actually be more of a problem now than in the past because the data aggregators like Nate Silver or HuffPollster are using statistical models to estimate the relative standing of the candidates, based partially on a combination of polling data but also using different assumptions and other added variables in their projections.

If you are interested in evaluating the bounce, you are best off looking at the polls from a single organization to see whether any shift occurs. In the current campaign, however, neither Gallup nor Pew are polling with the trial heat question and publishing results, so information from two of the most reliable data sources will not be available.

It is also important to remember that these are survey estimates, subject at least to sampling error, so any given bounce has to be at least 5 percentage points (assuming standard sample sizes) or larger to be confident that it involves a real movement in popular support. Across the last 20 conventions, the size of the bounce did not exceed this threshold six times (30 percent).

What has the bounce looked like recently?

While there are polling data that go back to the 1930s, it is best to look at the bounce since 1976, the first year in which both parties were operating under the current system of primaries and caucuses. This changed the fundamental role of the convention from a nominating event to a coronation event that became staged political theater.

In this period, there have been 10 pairs of conventions. The historical custom is for the party out of power in the White House to hold its convention first, so there can sometimes be an incumbency factor built in to candidate support levels. But the 2016 conventions have been scheduled in a somewhat different way than in the past: They are earlier and back to back.

Over this period, the bounce has gotten smaller — for the candidates from both parties. For the Democrats, the average absolute value of the bounce (a way to account for the two occasions when it was measured as -1 percentage point) across the last 10 conventions was 7.1 percentage points. In the earliest five conventions — Jimmy Carter's first to Bill ClintonWilliam (Bill) Jefferson ClintonDemocrats' choice: Unite or go down to defeat Biden says he has to do 'really well' in South Carolina Klobuchar says English should not be US national language, reversing from prior vote MORE's first — the average was 10.2; since 1996, it has been 4.4 percentage points. On the Republican side, it was 5.6 in the earliest period and 4 since.

In the first period, the candidate who was nominated first received a slightly larger bounce than the one who followed. In the last five conventions, the order hasn't made much difference in the magnitude, partially because the size has been reduced as well as the difference between the two candidates.

Why has the bounce gotten smaller?

There are a number of factors that can explain the diminishing size of the bounces. One of them is the elapsed time between the two conventions. In the first five, there was a month between the first and the second. In 1996 and 2000, the interval was reduced to two weeks. It went back to a month in 2004 — the contest between George W. Bush and John KerryJohn Forbes KerryNew Hampshire primary turnout is a boost to Democrats New Hampshire only exacerbates Democratic Party agita If Trump renegotiates Iran's nuclear deal, should it be a treaty this time? MORE — but since 2008, there has only been one week between the conventions, just as in this year's sequence.

Second, since 2000, the system of public financing of presidential elections in United States has completely broken down, and candidates are now raising and spending as much as they can without limits or federal assistance. The candidates have more money to spend than ever, and they are advertising sooner, including while the other party's convention is taking place. In addition to TV, they are deploying sophisticated social media campaigns on a wide variety of media. And they and their surrogates are responding in real time to charges and claims made by the other party.

In combination, this means that the first party to hold its convention — the party out of power — no longer has virtually complete control over the media agenda to hammer home their own message without rebuttal. So the process of consolidating the nominee's base is not as straightforward as it used to be.

What will happen in 2016?

The first bounce that journalists will look for and discuss will be Trumps's; it should be a topic on the Sunday talk shows. It probably will not be large because the Republican convention was not well organized to begin with. The Republicans also had a fair number of distracting news stories — commentary on the rollout of Gov. Mike PenceMichael (Mike) Richard PenceShort defends Trump's tweets as a 'very effective way' to communicate with Americans Trump's 'two steps forward, one step backward' strategy with China The state of the Democratic primary: Heading to a brokered convention?   MORE (Ind.) as the VP choice, the vote for the convention rules, Melania Trump's speech and Speaker Paul RyanPaul Davis RyanPaul Ryan says Biden likely won't get Democratic nomination Judd Gregg: Honey, I Shrunk The Party The Hill's Morning Report — Dems detail case to remove Trump for abuse of power MORE's (Wis.) speech, to name a few — that detracted from the central message around which the campaign will be organized. In addition, the party is still not unanimous in its support for Trump's candidacy, as witnessed by Sen. Ted CruzRafael (Ted) Edward CruzCruz 'impresses' his daughter with Chris Evans meeting Three Senate primaries to watch on Super Tuesday The advantage of paying for medical care directly MORE's (Texas) speech that did not include an endorsement.

Hillary Clinton has problems of her own in consolidating the Democratic base. Sen. Bernie SandersBernie SandersDemocrats redefine center as theirs collapses Speculation swirls around whether Bloomberg will make Las Vegas debate stage Pelosi: 'I'm not counting Joe Biden out' MORE's (Vt.) supporters will mostly return to vote for her, but probably not in sufficient numbers in the week or less in which the bounce will be measured to have much effect. So the standing of the candidates in the general election campaign is likely to start from about where it is today — a tight race with a slight lead for Clinton.

Traugott is professor emeritus of political science at the University of Michigan.


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