To get a perspective of the reliability of prior presidential polls we can see (click attachment below) that in 2004, every single major national poll underestimated Bush’s ultimate percentage win. However, only Marist and Fox News were completely wrong.
In 2000 the pollsters were generally more accurate. Most predicted Bush would win, but underestimated Gore ultimate percentage and exaggerated Bush’s final percentage.
Based on prior elections, it appears the most reliable national pollsters are TIPP [Investors Business Daily] and PEW Research for 2004. In the Bush vs. Gore 2000 race, Zogby, Gallop and PEW again were closest.