… Polls of so-called likely voters are almost always more favorable to Republicans than those that survey the broader sample of all registered voters or all American adults. Likely voter polls also tend to provide more reliable predictions of election results, especially in midterm years. Whereas polls of all registered voters or all adults usually overstate the performance of Democratic candidates, polls of likely voters have had almost no long-term bias.
This represents one difference between FiveThirtyEight’s Senate projections and several other forecast models. Our program “translates” registered voter polls to make them equivalent to likely voter results. With some exceptions, other forecast models do not. This is one reason the FiveThirtyEight model tends to show a more favorable outcome for Republicans. CONT.
Nate Silver, FiveThirtyEight