Proposition 1 was drafted by the state Legislature as a direct response to the U.S. Supreme Court's overturning of Roe v. Wade, which had established the federal right to an abortion for nearly 50 years.
"It's local government. It's states like California that are on the front lines of rights all across this country," Newsom told the roughly 500 people gathered for the rally at Long Beach City College. "Do not take that for granted."
He said the proposition on Tuesday's ballot is a chance "to send a powerful message all across America that we have your back." He said the overturning of Roe v. Wade could lead to other individual rights being stripped away, saying civil rights are "on the line."
Secretary of State Shirley Weber told the crowd such attacks on personal rights mean voters need to "pay attention."
"Freedom is not free," she said. "Justice demands constant attention, and forever shall we watch and be vigilant for it."
The rebuttal to the argument in favor of Proposition 1 in the Official Voter Information Guide distributed to voters by the Secretary of State's Office declares that "Proposition 1 is not needed to protect abortion rights but it will cost California taxpayers millions" through the increased number of abortion seekers from other states coming to California.
Eyewitness Newsmakers: Breaking down the 7 propositions on California's election ballot
"California law already allows access to abortion and contraception," attorney Heather Hacker said in the rebuttal. "But unlike state law, which limits late-term abortions unless medically necessary, Proposition 1 has no limit on late-term abortions."
Opponents have also argued that making California a safe space for abortion will result in a soaring number of out-of-state patients traveling to the state to seek the procedure "at the expense of California taxpayers."
More than 600 early voting centers were already open across Los Angeles County on Sunday, opening every day from 10 a.m. to 7 p.m. On Tuesday, Election Day, the voting centers will be open from 7 a.m. from 8 p.m. Voters can cast their vote or drop off their ballot at the locations.
Meanwhile, in Orange County on Sunday voters cast their ballots at a pop-up voting center set up outside the Santa Ana Zoo.
California's secretary of state said that of the 22 million mail-in ballots that had been sent out statewide, 3.8 million had been returned as of Friday. In Los Angeles County, the percentage of mail-in ballots submitted was 12.8%.
City News Service contributed to this report.