Two reasons why Texas's delegate allocation rules favor Obama.
Reason one:
A total of 126 delegates will be awarded based on the outcome of the vote in each of the 31 state senatorial districts.
But the number of delegates available in each district is not equal: Delegates are allocated based on the votes cast in districts in the 2004 and 2006 presidential and gubernatorial elections.
In the heavily urban, black districts of state Sens. Rodney Ellis of Houston and Royce West of Dallas, a good voter turnout in the past two elections means a combined total of 13 delegates are at stake in the two districts on Election Day.
Obama nationally has been winning eight out of 10 black voters, according to network exit polls.
But in the heavily Hispanic districts of state Sens. Juan Hinojosa of McAllen and Eddie Lucio Jr. of Brownsville, election turnout was low, and a combined total of seven delegates are at stake.
Clinton has been taking six of 10 Hispanic votes nationally. So a big South Texas win might not mean as much for Clinton as a big win for Obama in the two black districts.
http://www.caller.com/news/2008/feb/11/texas-arcane-delegate-system-suddenly-comes-play/I would note that it's not just the black areas of Houston which get more delegates based on this rule- other pro-Obama areas such as Austin will also benefit, as noted in this article:
"The delegate-rich districts are the most heavily liberal state senate districts. According to this calculation, they're in Austin and in two of the most concentrated African American parts of the state. Advantage: Obama."
http://marcambinder.theatlantic.com/archives/2008/02/texass_unique_primaucus.phpThe above article notes the second reason why the delecate allocation scheme favors Obama- there's a caucus at night that accounts for 1/3 of the delegates:
The process has two steps. First, folks vote. 126 delegates will be accorded proportionally via state senate district. Then, when polls close, they caucus in more than 1,000 precincts.
At the caucus, attendees chose the identity of the delegate and the presidential candidate that the delegate is supposed to represent. These delegates are sent to a "senatorial convention" a few weeks later, during which the final math is worked out and the actual delegate slate for the convention is chosen.
67 delegates will be chosen this way.