Hey, liberals: prepare now. Barack Obama will likely be losing when the pollsters try again after Mitt Romney closes out the Republican National Convention on Thursday night. He may well be back ahead, however, after his speech at the Democratic National Convention next week. My advice? Take a quick look at the polling averages right now — Pollster has Obama up by one percentage point — and then ignore the polling until a full week has passed after the Democratic Convention. By then, we’ll start to have a really good sense of what’s going on.

We’ve had a bunch of excellent summaries from political scientists in the last few days of how and why convention bumps happen, and what to expect this time around. We basically see two kinds of movement during the conventions: one temporary, and one permanent.

The temporary bounce is caused by genuine undecided voters, who may hear something they like and therefore poll as supporters of whoever is in the news, but will rapidly forget it and go back to being undecided. Again, there are relatively few undecided voters at this point, so that too shouldn’t be large.

The permanent change happens mostly because of voters who aren’t truly undecided. Some partisans who don’t pay very much attention to politics, or who do pay a lot of attention but for whatever reason have not yet warmed to their party’s nominee, will learn all sorts of good reasons for supporting him — and, as partisans, they’re generally open to the message they’re hearing. That will also be the case for some unhappy true independents or, if the incumbent is unpopular enough, disaffected supporters of the in-party. Once they click into the nominee’s column, they’re likely to stay there.

However, you’ll get a lot more of those in years when there’s been a divisive nomination which has left many partisans initially unhappy with their nominee. This cycle, most Republicans lined up behind Romney soon after the nomination was settled, so it’s unlikely there will be too much of that. Nor will there be very many Democrats who decide they won’t be voting for Obama and drift into the Romney camp.

The trick is that it’s hard to tell from the polling which category people are in, so the best thing to do is to just wait a bit until it all shakes out. Only then will we have a very good idea of what kind of presidential election we’re really going to have. But you should expect Romney to be ahead after this week, even if it will take at least another couple of weeks to see whether there has been any real change in the race.