they're weighted. Here's a theory put out about the 2000 race that suggests that Gallup was underrepresenting minority voters.
http://chris-bowers.mydd.com/story/2004/5/19/225039/641 "Likely Voter" Bias: Why Gallup was Wrong in 2000
by Chris Bowers
On November 6th, 2000, the final Gallup/CNN/USA Today tracking poll showed Al Gore at 45% and George Bush at 47%. This poll was similar to others conducted by different services in the final days of the election. During the same time period, NBC/Wall Street Journal showed Gore at 44% and Bush at 47%; ABC/Washington Post showed Gore at 45% with Bush at 48%. Tarrance showed Gore 41%, Bush 46%; Christian Science Monitor showed Gore 46% and Bush 48%. Among all final polls, only CBS (45-44) and Zogby (47-46) showed Gore ahead.
I am not a subscriber to Gallup, and thus I do not have access to old poll internals. However, if you are willing to take my word for it, in early October of 2000 I spent a couple hours after class one day pouring over the internals of the latest Gallup tracking poll in an attempt to better understand the state of the campaign. One discovery, which has stayed with me to this day, led me to conclude that they were underestimating Gore's support by around 2-3%, and overestimating Bush's support by the same margin. The discovery was that they had incorporated race into their model, and had done so in a way that appeared to more heavily weight white opinion than minority opinion.
In 2000, Gallup's national tracking model identified 87.5% of "likely voters" as "white" and 12.5% as "non-white." However, according to exit polls, only 81% of voters were "white" while 20% of voters were "non-white." If Gallup had incorporated an accurate model of minority turnout into their polls in 2000, their final tracking poll results would have been amazingly accurate. If only 12.5% of voters had in fact been "non-white" in 2000, then Bush would have won 50.13-46.13. Conversely, had Gallup projected that 20% of voters would have been "non-white," then their final poll results would have shown 47% for Gore and 46% for Bush. Perhaps not coincidentally, Zogby's Election Day numbers were 47% for Gore and 46% for Bush.
Although I lack access to internals from the time, I submit that Gallup and almost every other polling agency was wrong about Gore and Bush in 2000 because their polling models over emphasized white opinion. Further, it is entirely possible that the reason Zogby and CBS were more accurate than other polling services was because they incorporated an accurate model of minority turnout into their data. Considering demographic changes, it is quite likely that minority turnout will constitute an even larger percentage of the national vote in 2004 than it did in 2000. If, for example, the "non-white" vote will make up 22% of the national electorate in 2004, than even a model that assumes 20% minority turnout will be inherently inaccurate. Such a model would add nearly 1% to Bush and subtract nearly 1% from Kerry.