Don't trust Gallup polls.
Why you're right:
1. They poll too many Republicans. The most recent Gallup poll of about 1000 voters contained 40% Republicans and 31% democrats. According to a study by the Pew foundation, Democrats outnumber Republicans by about five percentage points in the actual electorate, just as they have done for the last decade. (Left Coaster, Pew)
2. Their likely voter model is flawed. Gallup's methodology systematically undervalues young voters, transient voters, immigrant voters and other groups likely to vote Democratic. A voter is given more weight if they answer yes to questions like "Have you ever voted in your precinct or election district?" (Emerging Democratic Majority)
3. They've been wrong before. For example, on 10/26/00 Gallup had Bush up 13 points over Gore. (Polling Report)
Why they're wrong:
Gallup claims their critics "don't understand the science behind the polls." But respected pollsters who understand the science of polls, like John Zogby, disagree with their methods. (USA Today)