Sensing opportunity, a band of former top Obama campaign operatives have just launched the most ambitious effort to date to loosen the GOP grip. Their goal: Make Texas competitive by the second half of the decade and eventually tip it for a Democratic presidential candidate for the first time since Jimmy Carter won here in 1976.

Led by former Obama field director Jeremy Bird, the Battleground Texas project plans to marshal much the same manpower and data-mining the Obama campaign used to swing states such as Colorado and Virginia in the past two elections…

Over lunch at a packed restaurant south of downtown San Antonio, Mayor Julián Castro beams at the Democrats’ chances in the years ahead. After giving the keynote address at the Democratic Party convention in September, Mr. Castro dropped into Virginia, Florida and Nevada as a campaign surrogate for President Obama. What he saw stunned him.

“The sheer intensity of the campaigning there, the all-out effort to find and mobilize votes, was unlike anything I have ever seen in Texas,” he said.

Mr. Castro contends the Republican room for growth in Texas is minimal. “But we Democrats,” he said, “we haven’t even begun to pick the low-hanging fruit.”