So how far ahead is Barack? Based on a report Josh Marshall thinks is fairly accurate, Obama leads Clinton now by about 110 pledged delegates, 1078 to 969 (2047 altogether). There are 796 unpledged delegates (superdelegates), which leaves 1206 left for the taking. A candidate needs 2025 to secure the nomination.
Since Democratic primaries are proportional, Clinton is now behind the 8-ball. To tie Obama, she needs 658 of the remaining 1206 (54.5%). Obama, on the other hand needs just 548 (45.5%). This is where proportional delegates gets tough. As long as Obama stays close to 50%, he doesn't even have to win Ohio and Texas to thwart Clinton. It gets worse. Since his childhood home state of Hawaii along with Wisconsin, where he leads in the polls, are up first, his delegate surplus should rise. Coming out of those contests, his margin is likely to be around 125--with still fewer delegates left for Hillary to win.
Ohio and Texas have a combined 334 pledged delegates at stake. A win of even 65% to 35% in those two states wouldn't catch her up--and no one believes she'll come close to that margin. If she wins at all, it will be modestly--say 55-45%. But that only makes up 34 of the delegates, and leaves her trailing by 90.
The math is now definitely in Obama's favor.