Before I get too far into this, I want to mention that I'm making a point which is explored by Jay Cost. Cost mentions that Hillary Clinton still has a shot at winning the nomination if she wins Texas and Ohio and Pennslyvania, but is still not the favorite to win. I want to take this a bit further and point out why Hillary Clinton does need both to win.
By the way, this diary is being addressed not only to supporters of Hillary Clinton, but to supporters of Barack Obama who continue to believe that if Hillary Clinton can eek out a tiny victory in two states where she went into as the heavy favorite, she will somehow become the favorite to win the nomination. March 4 is not make-or-break for Clinton, March 4 is limp-on or be finished. More to the point, Hillary Clinton needs decisive wins in Texas and Ohio, not to clinch the nomination or to even be considered the favorite for the nomination, she needs it to survive.
For Hillary Clinton, Texas and Ohio are what Iowa was to Barack Obama and, moreso, to John Edwards; a test of viability. Now, when I say this, that doesn't mean that John Edwards and Barack Obama were not viable before the Iowa caucus happened, nor does that mean that if John Edwards had won the Iowa caucus that he would've ultimately gone on to win the presidential nomination; however, neither could've had a reasonable chance of winning without it. The specific reason why John Edwards was no longer viable outside of Iowa was simple, he had no comparable campaign organization in any other state, nor could he expect to build one with the limited money he had. Obama's case is a bit different, because unlike Edwards, Obama did have substantial money to build campaign organizations in other states like New Hampshire, Nevada, South Carolina, and in many of the Super Tuesday states (and, he may very well have still won a couple of states like South Carolina, Georgia, and Illinois), but ultimately his money would've dried up and he'd become a non-factor in the presidential race (much like John Edwards did). Once Barack Obama won in Iowa, he became a viable candidate, and could sustain losses in New Hampshire and Nevada simply because he had proven his ability to win a state without a significant African American population. Also, Obama reaffirmed that viability with his massive win in South Carolina, and when he fought Hillary Clinton to a tie on Super Tuesday (basically, if both campaigns are able to make a credible argument that they won something like that, it's ultimately a draw), he had cemented his viability.
Hillary Clinton, on the other hand, did not need to win any of the early states in order for her to be considered viable, due in large part to her strong network in many of the larger Super Tuesday states and her name-recognition within the party. Hillary Clinton's strongest argument for her own viability was that no matter what happened, she could be guaranteed a large number of delegates on Super Tuesday. In a three-way contest, her strength on Super Tuesday would've probably put her into a position where she would've been considered the strongest candidate coming out of Super Tuesday (especially when you consider that Obama actually absorbed most of Edwards's supporters).
Since Super Tuesday, Barack Obama has win all eleven contests which have occured, and done so with double digit wins, which has given him a significant boost in delegates (Obama has racked up a 150 lead among pledged delegates because of these victories), while significantly improving his standing in the national polls, to the point where he is at worst in in a tie and at best winning by double-digits. All of this makes Obama, without a doubt, the frontrunner for the Democratic nomination.
What does this have to do with Hillary Clinton's viability? Well, the reverse side of Obama winning eleven contests by double-digits is that she has lost all of those contests by double digits. A lot of Clinton supporters have made note that Obama has done well because of demographics (African Americans and affluent whites), I believe they are correct, but the key difference between Obama and Clinton has been that Obama's shown an ability to make inroads with Clinton's constituency while Hillary Clinton hasn't been able to do the same with Obama's core group. Take a look at California's exit poll. Hillary Clinton beat Barack Obama by 10 points among voters at large, and she beat Obama by a massive 35 points among latino voters. If Hillary Clinton loses the popular vote in Texas, it will mean that Obama has annihilated Hillary Clinton's monopoly on the Hispanic vote (which seems to be apparant from Survey USA's latest poll). Also, notice that Obama's support among his key constiuency has not fallen, rather it has risen over time (among African Americans, Obama had gone from winning 75% of the vote, to nearly 90%). As many political scientists are probably aware, a successful coalition is able to retain its own base and make inroads with their opponent's base, this is what Hillary Clinton has failed to do, and it is why she is left in a position where she isn't viable. Ultimately, while Texas and Ohio victories are not sufficient for Clinton to win the nomination, they are necessary, and whether Hillary Clinton drops out after or not, the brutal truth will remain that, barring some extreme event happening, Hillary Clinton will not be able to win the nomination.