Who will be the next American president?

InDepth Editor Peter O’Donovan examines polling data in the lead up to the US presidential election to analyse which candidate is most likely to win the seat in the Oval Office


“Between Trump and Clinton, Clinton has consistently held a narrow lead, although in the most recent polls it has widened to an 11 point lead on average.”

After a year of bruising primary contests and dramatic revelations about the candidates running for office, the final stage of the American presidential election is drawing near with Hillary Clinton and Donald Trump as the two candidates representing the main political parties, the Democrats and the Republicans, respectively. The election itself is scheduled for 8 November 2016. The American presidency has been held by either a Democrat or a Republican since 1854, and based on polling figures it is highly likely that Trump and Clinton will be the only real contenders for winning the presidency.

However, this electoral cycle has seen an increase in support for the alternative parties (the main beneficiaries being the Green Party headed by Jill Stein and the Libertarian party headed by Gary Johnston). This is mostly a result of Trump’s campaign having alienated many traditional Republican voters, who have mostly gone to the Libertarian party, as well as Bernie Sanders’ primary run for Democratic candidate, which played a role in energising youth engagement in politics, some of which is now being directed into third parties. These parties may play a spoiler role in the contest between Trump and Clinton, as the American presidential voting system lacks the transferable vote feature of, for example, the Irish general election system.

Under a transferable voting system, on each count the candidate with the lowest number of votes is eliminated and their votes are redistributed according to the preferences that a voter indicates below their first choice vote. In the single winner plurality system used in the US, each voter simply gets a first preference and the candidate with the most first preferences on the first count wins. Since votes for third parties cannot be redistributed under this system, each vote for a third party is a vote potentially taken away from one of the main two candidates.

The graph below details how the candidates have polled nationwide in various polls conducted over the past month. As predicted, Trump and Clinton are consistently far ahead of their challengers, leaving it near certain that the two party system that currently prevails in the US will remain without serious challenge. Between Trump and Clinton, Clinton has consistently held a narrow lead, although in the most recent polls it has widened to an 11 point lead on average. This is likely the result of the release of tapes from 2005 in which Trump made degrading comments about women and bragged about groping women to Billy Bush, of Access Hollywood fame. This makes it unlikely that Trump will claw back enough support to win the presidency by the time that voting actually opens. Note that the trend lines for the third party candidates are disjointed because some polls only asked voters about which of the two main party candidates they preferred.


Trump has claimed, in response to his poor polling figures, that he will get better results on the day of the election than his polling figures currently indicate. This argument references what is known in American politics as the Bradley effect, named after former Los Angeles mayor Thomas Bradley. Bradley, an African American, ran for governor of California in 1982 and was ahead in the polls before the election but lost to his white opponent, George Deukmejian, when the votes were actually cast. Similar results have been seen in various other American elections in which a black candidate ran against a white candidate, and political analysts have theorised that voters may have claimed they were going to vote for the black candidate on the basis of it being seen as socially desirable to do so, but then cast their ballot for the white candidate. Trump is effectively arguing that some of his supporters pretend not to support him in polls to avoid stigma potentially associated with being a Trump supporter.

The results from the primaries do not bode well for Trump’s chances of pulling off a Deukmejian-style reversal on the day of the election. While Trump did outperform his polling numbers by an average of 2.4% on the day of the primaries, it is important to remember that these polls include voters who are undecided on the day of the poll who do end up deciding to vote for someone, leading to most candidates doing better in elections than in polls. For comparison, Ted Cruz got an average of 2.5% more of the vote in the primaries than polling figures had previously indicated he would.

“The final result of the presidential election is determined by the electoral college – this is the system by which each state has a number of votes in the final election, roughly proportional to the population of the state.”

Furthermore, Trump’s theory that people who will eventually vote for him are claiming they won’t on the basis of social acceptability are in contradiction with the results obtained by different styles of polling. If this were the case, one would expect Trump to do better in anonymous online polls as opposed to phone interviews where there is another person directly listening to the voter’s responses and possibly making judgments on them. However, polls actually show the opposite result – Trump has actually done better (average 40.1% support) in polls over the telephone than he has in anonymous online polls (average 39.2% support). Based on these results, it seems unlikely that a “shy Trump voter” effect will be seen on the day of the actual election.

It should also be borne in mind when comparing Clinton and Trump’s polling figures that due to the structure of the American presidential election, the overall share of the votes won by each candidate across the nation as a whole are less important than the way in which the voting breaks down by state. The final result of the presidential election is determined by the electoral college – this is the system by which each state has a number of votes in the final election, roughly proportional to the population of the state. Most states give all of their votes to the candidate who gains a majority of the popular vote in the election in that state.

Hence, it is much more important to win states than to win an overall majority of votes. This is also why so-called swing states, which don’t consistently vote for Democrats or Republicans, are so important. In the last three US presidential elections, the ultimate result was determined by the results in Florida, Pennsylvania and Ohio. In recent polls, both Florida and Pennsylvania are favouring Hillary over Trump (by three points and eight points respectively), while Ohio has been swinging back and forth, although the most recent poll (on October 9) showed a four point lead for Clinton. The less influential but still important swing states of North Carolina, Virginia, and Nevada have also shifted back and forth as the election cycle has continued, but recent polls suggest that Clinton is slowly consolidating a narrow lead in each. The other more minor swing state, Colorado, is showing a 12 point lead for Clinton.

In conclusion, it looks likely that Hillary Clinton will not face a very strong contest from Donald Trump on the day the votes are cast. Concerns over Clinton’s health after she “fell ill and may have fainted” during a 9/11 memorial service have been overshadowed by Trump’s insulting comments about sexual groping in the recently released tapes. While anything could happen between now and polling day, barring a major upset, Hillary Clinton’s path to being the first female president of the United States of America now seems clear.