New national poll from the Los Angeles Times/Bloomberg (August 16-18, 1,248 RV, MoE +/- 3.0%) shows Obama's lead shrinking to just two points, down from a 12 point lead in June:

Obama 45 (-4)
McCain 43 (+6)

Overall, Obama leads by 3.0% in the RCP National Average.

Notable quote from the LAT article:

More striking than the head-to-head matchup, however, is the drop in Obama's favorable rating in the run-up to his selection of a running mate and the Democratic National Convention next week in Denver.

Obama's favorable rating has sunk to 48% from 59% since the last Times/Bloomberg poll in June. At the same time, his negative rating has risen to 35% from 27%.

By comparison, McCain's ratings have hardly budged during the same period: 46% of voters have a positive feeling about him; 38% give him negative ratings.

"All the negative attacks from the McCain campaign seem to have been paying off," said Times Poll Director Susan Pinkus, who oversaw the survey.

Overall, Obama's lead over McCain in the favorable/unfavorable ratings is down to just 1.8 points. Obama has a net plus 15.0 favorable rating, while McCain is at a net plus 13.2.