[W]e’ve still got four months until the election. Four months in which we’re going to be covering the campaign also nonstop. So help me out here: What’s going to matter between now and then?To begin with: what John said about House and Senate and some downballot campaigns. The outcomes are important, the campaigns are meaningful, and as a bonus they're often excellent fun. We're going to get events that change things -- within the last few days, the Nevada Senate contest tipped, perhaps decisively, against the Democrats, while in Arkansas a Democratic House candidate dropped out with scandal whirling around him.
About the presidential campaign, the first point I'd make is that things will happen that have nothing to do with electoral outcomes but are still important, especially for whichever side wins. Both candidates have already made a number of policy commitments. They'll make more. They'll also make choices about which ones to emphasize, and the more visible a commitment is, the more it tends to constrain the winner after the election. Think Barack Obama's threshold for tax increases -- or his choice to push for health care. Politicians can break those types of promises, but there are costs, and so what happens in the campaign tends to set up what they actually do in office.
That's not all. Representation also includes non-policy promises: politicians promise how they'll behave, and even in a way who they will be. Those promises, too, constrain candidates once they get elected. For example, Bill Clinton promised a style very different from what he claimed was an out-of-touch George H.W. Bush, and he spend a fair amount of time and energy after the election attempting to keep that promise. A lot of those sorts of promises have already been made, but again the more visible during the fall campaign, the more it will be remembered afterwards.
But I suppose what the question was really about was the outcome of the election, so I'll get to that too, although John covered most of it. Here's how I'd break it down:
Potentially major effects: those would be things that would change the fundamentals of the race. Most likely that would mean either a major surge or a major disaster in the economy. It's also possible, however, that a national security event or even some wildcard event could change things.
Potentially minor but real effects: see, this is where it gets tricky, because there are lots of them. John mentioned ad campaigns and organized mobilization. I'd add that a good VP pick could help Romney a point or two in his or her home state; a disastrous pick could cost a couple of points nationwide. Aggressive voter purges, voter ID laws, and other such measures may or may not be enforced harshly (and some of these are still in the courts or otherwise still contested), and that could mean a chunk of voters, which might be significant in very close states. If the election winds up very close, it's possible that one of the parties will open up an electoral college advantage, although I wouldn't even think about that until the last few weeks -- Nate Silver is great at tracking that sort of thing. Late-breaking campaign events might make a small difference, just the way that TV ads might make a difference; a good example was the late-breaking campaign finance scandal in 1996, which may have cost Bill Clinton a couple of percentage points.
Less likely to have even marginal importance on election outcomes: convention speeches; the debates; gaffes, ads, and campaign events before the last few weeks of the campaign.
I'll toss in one wildcard. Ideological extremism hurts candidates. Perceptions of extremism seem to be showing up in polling of the Republican Party, and as expected hurting them. However, so far Mitt Romney seems to have created a perception of moderation that is, I would say, fairly removed from his issue positions. If that was to really change, that could be make a real difference, it seems to me. However, I think it's unlikely that it will change; Romney has every incentive to portray himself as a non-ideological technocrat, and it's unlikely that his issue positions would be sufficient to change general perceptions. Still, it's something worth keeping an eye on, I suppose.