Will 90 percent of Sanders supporters really vote for Clinton?
© Getty Images

The Pew Research Center issued two reports in the last week that demonstrate the difference between intraparty and interparty contests as well as the analytical power of a panel survey design and analysis. Last week, they described shifting preferences among Republicans in the primaries and for the general election, and on Monday they did the same for the Democrats.


The big headline from Monday's press release is that 90 percent of "consistent" supporters of Sen. Bernie SandersBernie SandersOcasio-Cortez's 2nd grade teacher tells her 'you've got this' ahead of DNC speech Trump and allies grapple with how to target Harris Chris Wallace: Kamala Harris 'not far to the left despite what Republicans are gonna try to say' MORE (Vt.) among Democrats and independents leaning Democratic favor Democratic nominee Hillary ClintonHillary Diane Rodham ClintonThe Hill's Campaign Report: Biden, Harris make first public appearance as running mates Trump campaign spox rips GOP congressman over rejection of QAnon conspiracy Biden hits back after Trump's attacks on Harris MORE over GOP nominee Trump, with only 8 percent saying they would support Trump. However, the release did not compare the conversion rates between the two reports.

Last week, Pew reported that 79 percent of Republicans and independents who lean Republican said they would vote for Trump over Clinton, while only 53 percent in the group are "certain" they will. This point illustrates that in the primaries and caucuses, allegiances may vary, but most partisans will return to support their party's nominee in the general election.

And at this point in the campaign, Clinton is relatively better off than Trump among their respective partisans who favored another candidate. While there is variation in voters' preference among candidates within one party's primaries and caucuses, when it comes time for the general election, most of them have historically returned to support their party’s nominee.

Because Pew has a panel design in which they interview the same respondents multiple times, they can assess the total level of change of voters in the electorate rather than the net level of change that comes from comparing repeated cross-sections containing different samples. This is the same analysis issue that arises when measuring presidential approval in repeated cross sections: We know whether President Obama's approval went up or down in total, but we don't have the measure of how many people went from approving to disapproving or from disapproving to approving across the two polls. Pew's design makes this possible.

The Pew analysis covers three polls conducted in December 2015, March 2016 and April 2016. Only 20 percent of the Democratic respondents consistently backed Sanders in all three surveys, compared to 29 percent who consistently favored Clinton. Almost half (44 percent) switched their preference at least once in this period. On the Republican side, only 34 percent of the Republicans consistently favored one candidate across the three surveys, of which 23 percent were consistent supporters of Trump across this period. That means that two-thirds changed their preference at least once.

These patterns are a reflection of a number of factors, including the fluidity of preferences in the primaries as voters learned who the candidates were and what they stand for. Across the six-month period, the winnowing process is severe as the respective fields of candidates get reduced within their respective parties. Since the Democrats started off with only four candidates, the switching level was slightly lower than on the Republican side, where they started off with 17 candidates.

The big unknown for the general election is what the turnout levels will be in November. In each party, there is a visible minority of partisans who supported other candidates and who are not enthused about their party's nominee. Will they come around? Or will they end up staying home? In the tight contest we expect, as reflected in the current polls, turnout can make the difference. For both Donald TrumpDonald John TrumpNew Bob Woodward book will include details of 25 personal letters between Trump and Kim Jong Un On The Money: Pelosi, Mnuchin talk but make no progress on ending stalemate | Trump grabs 'third rail' of politics with payroll tax pause | Trump uses racist tropes to pitch fair housing repeal to 'suburban housewife' Biden commemorates anniversary of Charlottesville 'Unite the Right' rally: 'We are in a battle for the soul of our nation' MORE and Hillary Clinton, winning over their uncommitted partisans will be the key to the outcome.

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

The views expressed by contributors are their own and not the views of The Hill.