Here's Pollster.com's summary of the state of the race on the eve of super-duper-enormous Tuesday. There are kind of two ways you can look at this chart. One is that Clinton's had about the same level of support forever and now that it's a two person race the undecideds will break against the de facto incumbent and Obama wins. Another is that Clinton's maintaining a small lead and will probably secure a narrow victory tomorrow that takes the wind out of Obama's sails and leaves her victorious.
As is frequently the case in America's oddly arbitrary candidate selection process, an enormous amount hinges not on the objective results tomorrow but on the reporting of the results. The ambiguity between the results viewed as a race for delegates, as a race for states, and as a race for the semi-national Feb 5 popular vote only increases the extent to which basically made-up media narratives will be very important. Given that he usually gets good press, Obama probably has the edge in terms of winning a spin war in the event of an ambiguous outcome.
Matthew Yglesias is a former writer and editor at The Atlantic.