To answer Mr. Chait's question, Texas and Ohio matter more than the tweener contests beforehand because those are the states that Senator Clinton is making her stand. To remain the de facto frontrunner, Obama not only has to maintain a lead in terms of elected delegates, he has to show at some point that he can win a race in a state where the battle has been joined.
Having lost New Jersey, Massachusetts, California, and arguably Florida and Michigan to boot, he needs to win a big, urban state at some point to make the case that his political reach extends beyond the retail skills he has demonstrated in the smaller states and in the caucuses. Losing in Ohio and Texas would show he can't deliver the knock-out punch, and that he can't win the Big One; moreover, it would give Clinton the momentum going into the remaining contests, particularly Pennsylvania and Wisconsin. Since it is in the larger states that a disproportionately high number of SuperDelegates are situated, he can't afford to continue the trend of winning only the easy battles and losing all the contested races in large states.