The Valley is not, however, the richest source of delegates. Of the 193 delegates who will be pledged to support a candidate at the national convention in Denver, 126 will be allocated proportionally according to how candidates finish in the state's 31 state Senate districts. That means a second-place finisher could receive some delegates.
Not all Senate districts are equal: Those with a history of high Democratic turnout will award the most delegates. The biggest prizes among them are two predominantly black districts in Houston and Dallas, where the state senators back Obama, and another in Austin.
Sixty-seven pledged delegates will be chosen in a nominating process that will begin with precinct caucuses on the evening of March 4. That requires an unusual degree of organization by campaigns, which must turn out voters for the primary and for caucuses, and by voters, who must produce receipts showing they voted in the primary to participate in the caucuses.
While early voting in the state's 15 most populous counties is up nearly five-fold over the presidential primary in 2004, records kept by the Texas secretary of State show early voting is almost 10 times stronger than it was four years ago in the black strongholds of Harris and Dallas counties.
There are 35 elected officials and party leaders, known as "super delegates," who are free to vote for any candidate. Clinton has the support of 12 Texas super delegates, and Obama has five.
"They love her in my district," says U.S. Rep. Solomon Ortiz of Corpus Christi, one of a number of veteran Texas Democrats backing Clinton.
Some Democrats who represent other parts of Texas detect a different attitude among their constituents — one that drives them towards Obama.
"He won't be a drag at the top of the ticket," said Jim Dunnam, the Democratic leader of the Texas House. He hopes a five-seat shift in November will give his party the state House majority.
U.S. Rep. Chet Edwards, who represents a Republican-leaning district that includes President Bush's ranch, endorsed Obama in part because he fears Clinton will motivate Republicans to go to the polls.
"Hillary Clinton has been an effective senator and a good person. She doesn't deserve the demonizing," Edwards said. "But the reality in Texas is, many voters already have cast their judgment."
Contributing: Paul Overberg in McLean, Va.