All but one of the late surveys shows Clinton leading by margins of 5 to 13 points, so to no one's surprise, most expect Hillary Clinton to defeat Barack Obama tonight. The suspense seems to be about the size of Clinton's margin. On that score, unfortunately, the polls are not conclusive.

Why not? Here's the short version: (1) The pattern of smaller undecideds correlating with larger Clinton margin has largely disappeared over the last week, (2) tracking polls have been inconsistent about late trends and (3) the ultimate margin will depend on how well these surveys have selected likely voters. The longer version follows:

1) Do undecideds look like Clinton voters?

Maybe, maybe not.

[. . .]

However, as I look at the pattern of undecideds in the most recent SurveyUSA and Quinnipiac surveys, I see no clear pattern in the undecided either by region or demographic subgroups. On the Quinnipiac survey, for example, the percentage of undecided voters is roughly same among African Americans (6%) and white voters without a college education (5%).

[. . .]

2) Are polls showing a late trend?

Once again, unfortunately, the bottom line is maybe, maybe not.

The Zogby rolling average tracking shows the Clinton margin growing from one percentage point (46% to 45%) to ten (51% to 41%). However, other surveys that have tracked twice over the last week to ten days show no consistent trend.

[. . .]

Our trend estimates at another wrinkle. The standard trend lines look parallel, suggesting little or no change in Clinton's 6-7 point margin over the last week. However, as Charles Franklin explained earlier this morning, the more sensitive estimate -- which gives greater weight to more recent polls (including a few not that had not been tracking a week ago) -- shows a slightly bigger Clinton margin (8.4 points).

So, again unfortunately, we either have evidence of a late trend, or we do not.

[. . .]

3) "Its the Turnout Stupid"

Thats the way FiveThirtyEight's Poblano put it yesterday, and he's right. For all our worry about late shifts and the problems of interpreting the "undecided" category, the collective accuracy of the polls (or lack thereof) in predicting Clinton's probably depends even more on how well they have done selecting "likely voters."

Mark has done a great job telling us we will know this evening.

As to Mark's point about turnout, My RedState colleague, Erick Erickson, posted a little anecdotal data to keep us political junkies salivating until the polls close. So far it sounds like advantage Hillary.