After Wisconsin, the critical March 4 states are Ohio and Texas. Currently, polls show Senator Obama trailing Senator Clinton by ~15% and ~7% respectively. Texas is getting tighter - and the primary/caucus hybrid there could put Senator Obama within striking distance of Senator Clinton, at least in terms of delegates.
However, I think Ohio should not be written off - apart from the reason that Senator Obama has not yet campaigned in Ohio. There are quite a few similarities between Ohio and Senator Obama's home state of Illinois. For starters, Illinois and Ohio are both midwestern states. Major agricultural products in both states include corn, soybeans, dairy products, cattle, and hogs. Common industries include machinery manufacturing, processed foods, and rubber products. So Senator Obama should be tuned into the needs of Ohio's working-class voters.
In addition, steel is a major industry in Ohio - and Senator Obama worked with folks who lost their jobs after the steel industry collapse of the '70s and '80s, so he should be able to relate with similar folks in Ohio.