SAN ANTONIO — Rebecca Contreras was planning to vote again for President Joe Biden, but the 30-year-old Texan was uncertain he could win. Now with Vice President Kamala Harris as the de facto nominee, she said her confidence has crept back.

“Maybe there’s some hope again, and our votes can matter,” the San Antonio social media marketing specialist, who considers herself progressive, said Monday. 

Long seen as reliable Democratic voters, the support of younger Latinos like Contreras…