Obama's Decisive Bump

President Obama’s convention bounce shows no sign of subsiding. Yesterday’s Fox News poll shows him with a five-point lead over Romney among likely voters—48 to 43 percent—and he continues to lead in the Gallup tracking poll, which shows him with a six lead over the Republican nominee, 50 to 44 percent.

It’s hard to overstate how dangerous this is for Mitt Romney’s bid for the presidency. Conventions are one of the few points when candidates can shift the race and make meaningful gains. This makes sense—they’re little more than long, effusive advertisements, broadcast by major media outlets and seen by tens of millions of Americans. But while conventions can do a huge amount to change the dynamics of a presidential race, things tend to stabilize afterward. In their new book The Timeline of Presidential Elections, political scientists Robert Erikson and Christopher Wlezien explain the extent to which conditions harden in the post-convention period:

It is easy to imagine that many voters shift as a result of the conventions, and the evidence indicates they do. […]

The vote margin coming out of the conventions is different than that going in. And if we measure the consequences a few weeks after the dust of the final convention settles, the result is a decisive bump in the polls for whoever had the best convention—not a fading bounce.

You can see hints of this in polls that show little difference in results among registered voters and likely voters. The Democratic National Convention may not have convinced undecided voters to support President Obama, but it energized his supporters and closed the enthusiasm gap between the two parties.

If you look at the last 30 years of Gallup polling, one trend becomes clear—the leading candidate after the conventions almost always goes on to win the election. This, of course, isn’t to say that the election is a lock for Obama, but that Romney’s odds have become longer than they actually look.

There’s a chance that the former Massachusetts governor will be able to make up the deficit in the debates, but I have my doubts. The consensus among political scientists is that the presidential debates have little effect on the final outcome of a race. In a piece for Washington Monthly, John Sides quotes political scientist James Stimson: "There is no case where we can trace a substantial shift to the debates." At most, Sides notes, debates can nudge a very small share of the public in close elections. The best predictor of where a candidate will stand after the debates is where he stood before the debates.

In other words, if Mitt Romney can’t pull ahead by the beginning of October—or at least, make the race a toss-up—then the only accurate assessment you can make is that he’s likely to lose.

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