CLEVELAND — Presidential nominating conventions put political scientists in an unusual position. Most of the time, we advise journalists and the public that campaign events matter less than they think. The minutia of the campaign, including many stories that political reporters cover for days or even weeks, almost never detectably effect the election result.
Yet conventions are different. In contrast to the rest of the campaign, political scientists have good evidence that the conventions change public opinion. Two of the best analyses of how polls have changed over the course of presidential campaigns are Robert Erikson and Christopher Wlezien's book The Timeline of Presidential Elections, and Jim Stimson's book, Tides of Consent. Both show that there are often large poll swings before and during the two conventions. After the conventions are over, the polls are much more stable and predictive of the election result. As a result, the candidate leading in the polls immediately after both conventions has won the popular vote in every election since 1952.
Why do polls move so much before and during the conventions and so little after? Conventions consolidate and energize the supporters of each party. People who see themselves as Democrats or say they are independent but usually vote for Democrats come home during the Democratic convention, while the same thing happens on the Republican side. Sometimes one (or both) of the parties can't convince all of its typical supporters to vote for its candidate. Even in those instances, polls become predictive after both conventions because almost all the consolidation that will happen that year has happened by that point.
Fewer people watch the conventions on television than they did in decades past. But the polls continue to stabilize after the conventions in similar ways, probably because messages from the convention still filter out through news coverage, personal social networks, and social media. Party supporters are still reminded why they like their party and dislike the opposing one.
There are many ways to evaluate a convention. But a key metric is whether the convention achieves its most important task: consolidating the party for the fall. This Republican National Convention in Cleveland is novel because it is nominating Donald Trump, an unusual Republican. The key question is whether it can still rally all Republican sympathizers behind him.
Given that, Rudy Giuliani's speech has to be rated as a good one for Trump as a candidate. Giuliani is well-known and popular with Republicans all across the country. He gave a very aggressive, militaristic speech, quite similar to the speech he gave supporting George W. Bush at the 2004 RNC. Giuliani's provides a message Republicans are used to hearing and signals that Trump is a standard Republican.
There has been lots of talk over the past two days about portions of Melania Trump's speech being plagiarized from Michelle Obama's 2008 Democratic National Convention speech. In the big picture, I don't think this plagiarism will hurt Trump much. Plagiarism is a serious thing, but reporters, academics, and speechwriters care much more about plagiarism than most people. Also, to people who aren't following closely, coverage of this can easily seem like the media or Democrats are attacking the candidate's spouse, which could easily offend Republican voters and lead them to identify with her.
Mitch McConnell and Paul Ryan spoke Tuesday night. That the Republican leaders in Congress appeared in Cleveland is helpful to Trump. On the other hand, the speeches were much less helpful than they could have been because they mentioned Trump very little. It would have been better if they gave more direct, personal endorsements.
What's really hurting Trump this week is the many Republican politicians who failed to appear at all. The president of the UFC, the president of the Trump Winery, professional golfers, 1980s television stars, and many members of Trump's family (in addition to the candidate's spouse, who usually speaks), don't directly hurt Trump, but they don't help much either. The fact that various other obscure figures are speaking instead of established or up-and-coming Republican politicians from across the country is a lost opportunity to send a message to the public that Trump is the candidate all Republicans can and should support.
The biggest problem at the RNC so far is all the Republicans who stayed home. Speeches endorsing Trump from John Kasich, Jeb Bush, Mitt Romney, or pretty much any blue state Republican senator or governor would have been very beneficial to Trump. But these Republicans have declined to attend or speak on Trump's behalf, reducing his ability to consolidate Republican voters before heading into the fall campaign.
Usually, if the nominee can't consolidate part of the party base during his or her convention, that portion cannot be won over at all. The convention is the candidate's best chance. While this convention has been disorganized and had some missteps in the programming, it is the Republicans who never showed up in Cleveland who are hurting Trump most of all.