Prop 40 Election Results: California's billionaire tax

OAKLAND, Calif. - Election Day is in full swing across California and the U.S. with voters casting ballots in key races, including the closely watched Proposition 40, which would enact a one-time tax on billionaires.

Ballot drop-off boxes opened Oct. 5 around the state, and will remain open through 8 p.m. on Nov. 3. Ballots can also be returned by mail or to a county election office. Mailed ballots must be postmarked on or before Nov. 3. Reminder: if you're mailing your ballot on Election Day, state officials recommend having a postal worker manually postmark your ballot at the post office.

What is Prop 40?

The backstory:

Prop 40 has been one of the state's most-debated ballot measures in the midterm election – with supporters like Sen. Bernie Sanders, Rep. Ro Khanna and California Superintendent of Schools Tony Thurmond, and surprising opponents like Gov. Gavin Newsom and rival gubernatorial candidates Xavier Becerra and Steve Hilton.

If passed, Prop 40 would allow for a 5% one-time tax on Californians who have assets valued over $1 billion, including investment assets, private business interests and retirement funds.

The tax revenue earned by the ballot measure would go toward state health care, food assistance and education programs.

Track live election results as they come in below.

Polls close at 8 p.m. on Tuesday.