After more than four years of planning, study and political debate, a proposal to build a $2.3 billion reservoir in Santa Clara County — the largest reservoir constructed in the Bay Area in more than 20 years — will reach a make-or-break moment Wednesday.

The California Water Commission, a 9-member panel appointed by Gov. Gavin Newsom, is scheduled to vote on whether the project, which would be located near Pacheco Pass, will continue to be eligible to receive $496 million in state funding.