SACRAMENTO, California — Gov. Gavin Newsom’s signature mental health bond was too close to call Tuesday night, with support hovering over 50 percent around midnight while ballots were still being tabulated.

Proposition 1 would change the Mental Health Services Act, a 2004 tax on incomes over a million dollars that currently generates around $4 billion annually, imposing new requirements on how counties report and spend the funds on mental health programs. The measure also includes a $6.4 billion bond that supporters say would build 11,000 addiction and mental health treatment beds and supportive housing for veterans.

In a poll released a few days before the election, half of likely voters said they would approve of the measure. Pollsters and campaign officials have predicted historically low turnout, which hurt the proposition’s chances.