AUSTIN — Texas was not a state that played to former Vice President Joseph R. Biden Jr.’s strengths, especially for a campaign that, four days before the Texas primary and without a significant win, looked positively weak.

Large and expensive, a proper effort in Texas would stretch the resources of his cash-strapped campaign. With a large Latino base in the Texas Democratic electorate, the demographics were supposed to favor Senator Bernie Sanders of Vermont. And the $48 million in ads raining down from Michael R. Bloomberg, the former mayor of New York City, was likely to steal more votes from Mr. Biden than from any other candidate.

Yet with unwavering support from black voters in the cities and a surge in the suburbs, Mr. Biden notched his most significant win of the primary calendar here with an early-morning call on Wednesday, netting him a large and unexpected share of Texas’s 228 pledged delegates in the third-biggest state in the Democratic primary.

The candidate made his Texas targets clear with his travel the day before Super Tuesday, spending his final 24 hours of campaigning in Houston and Dallas, two cities with large black populations and multiple congressional districts that could help build a delegate total. Final exit polls showed Mr. Biden outperforming Mr. Sanders among black voters by 60 percent to 17 percent, a margin that most likely helped offset Mr. Sanders’s 45 percent to 24 percent advantage among Latino voters.