Here are some big caveats:
- Sampling error: even a flawless poll can calculate a percentage only within a certain range, or confidence internal. (News reports often call it the "margin of error.") That is, the actual figure may vary by a couple of points either way: see here for a thorough explanation.
- Other survey errors: "Unfortunately, there are several other possible sources of error in all polls or surveys that are probably more serious than theoretical calculations of sampling error," the Harris survey has said. "They include refusals to be interviewed (non-response), question wording and question order, interviewer bias, weighting by demographic control data, and screening (e.g., for likely voters). It is difficult or impossible to quantify the errors that may result from these factors." The increasing use of cellular phones has made it tougher to get a representative sample. It is possible, though unlikely, that such errors are consistently biasing polls in favor of one candidate or the other.
- Undecideds and switchers: A few voters are still undecided and some "decided" voters could switch. "We still have 11 percent of the sample saying 'we could possibly change our mind,'" says Pew president Andrew Kohut. "This is our projection, and our projections have been pretty good, but there's always the possibility things could change."
- Sandy: A previous posts have indicated, the aftermath of Hurricane Sandy could depress turnout in the heavily-Democratic New York metropolitan area, cutting into President Obama's national popular vote total.
- Romney wins: a late surge in Pennsylvania could give Romney a 277-261 victory. The latest Ohio poll is within the margin, so it is just possible that Romney could win there, too.
- Obama wins by a bigger margin: victories in Virginia, Colorado, and Florida could make it 337-201 in Obama's favor.
- A popular-electoral vote split: as noted, Sandy could tip the popular vote to Romney without switching any states. Obama would have the presidency, but Republicans would have a talking point.
- A tie: given trends in key states, the possibility of a tie in the electoral college seems pretty remote -- but it is not quite at zero.