National polls in the presidential campaign are mainly useful as barometers of public sentiment, rather than reliable predictors of who will win. As a barometer then, there’s a storm warning for the McCain campaign in the new numbers from the Gallup Daily Tracking poll. Barack Obama has opened up a double digit lead — 52 percent to 41 percent — over McCain for the first time since Gallup began daily tracking in March.

Interestingly, the period covered in the poll is Oct. 5-7, so the result does not reflect changes in voters’ sentiments after the debate on Tuesday, which Obama won, according to most polls and pundits. (In the CNN snap poll after the debate, only 30 percent of respondents liked McCain’s performance, as opposed to 54 percent who thought Obama won.)