The Republican and Democratic national conventions bring the possibility of change to this static presidential race. But how will it change, and how much? Here is some of what we know.

The party conventions typically cause the largest swings in the polls during the presidential campaign. During the convention season, the polls are unstable and likely to change more than at any point afterward, according to research by the political scientists Robert Erikson and Christopher Wlezien.

As is well-known, a party convention typically creates a “bump” or “bounce” for the candidate being nominated. Across presidential elections from 1964 to 2008, this has typically meant a 5-6 point increase, according to both political science research and the tabulations of pollsters like Gallup.

This leads to a first question: Why a bounce at all? Why would the conventions have this effect in the first place? After all, they are largely symbolic events, lacking in real drama and thus covered less and less by news organizations and watched by fewer and fewer viewers. (This paper by political scientist Costas Panagopolous documents the decline.)

One reason is that conventions generate a larger “dose” of information than is provided by the daily ebb and flow of news coverage. Thomas Holbrook’s study of campaigns from 1984-92 found that front-page coverage of each presidential candidate rose sharply during his party’s convention.

A second reason is that the news coverage during the convention favors the candidate being nominated. There are typically no increases in the coverage of the Republican candidate during the Democratic National Convention, and vice versa. Thus, unlike the later debates, a convention allows the candidate to present himself, Mr. Holbrook writes, “in a relatively uncontested format.” So conventions typically generate not just news, but favorable news for the candidates.

Next question: How big is the bounce? The bounces have ranged from essentially zero (John Kerry in 2004) to about 14 points (Barry Goldwater in 1964 and Bill Clinton in 1992). Two things appear to affect the size of the bounce, according to Mr. Holbrook in a recent blog post. One is how well the candidate is doing, relative to expectations. Here, we can think about expectations in terms of the underlying political and economic fundamentals. So a candidate running behind what the economy and other fundamentals would predict should get a larger convention bounce. A second factor is the timing of the conventions. Traditionally, the party whose convention occurs first — the party currently not in control of the White House — has had the larger bounce, which makes sense if the challenger is not as well-known as the incumbent and therefore voters have not formed as strong an opinion about the challenger. However, holding the conventions close together and somewhat later in the campaign season, as in 2008 and 2012, seems to mute the advantage of going first.

Final question: Will the bounce persist? Will the changes during this period have lasting benefits for President Obama or Mitt Romney? The term “bounce” makes it seem as if the candidates numbers will go up and then down, rendering any impact temporary. In fact, the conventions tend to leave a more permanent imprint. Mr. Erikson and Mr. Wlezien in their study of the 1952-2008 presidential elections find:

On average, the party that gains from before to after the conventions maintains its gains in the final week’s polls…Although the convention season is the time for multiple bounces in the polls, one party ends up with an advantage when the dust clears.

Of course, this is an average result, not a description of every election. In 2008, Mr. Obama was behind in several polls after the Republican convention, only to cruise to a comfortable victory.

What does this mean for 2012? I’ll leave the exact forecast to Nate, but I would expect only small bumps for either party. Neither candidate is really polling above or below expectations at the moment. The fundamentals going into the race suggest an Obama victory, but by a relatively small margin — and that’s exactly where the polls are right now. Moreover, scheduling the convention late in the summer and back-to-back should mitigate their impact. And there is only a small number of self-described undecided voters, which may help explain why another high-profile event, the naming of Representative Paul D. Ryan as the running mate, has not really moved the national polls.

To be sure, even “small bumps” might be enough to put the race on its head, moving Mr. Romney from a slight underdog to a slight front-runner. If that proves true, the question is whether Mr. Romney will sustain that lead through Election Day.